All graphs for this bug:
  https://chromeperf.appspot.com/group_report?bug_id=842755

(For debugging:) Original alerts at time of bug-filing:
  https://chromeperf.appspot.com/group_report?sid=125b14bbce90e137c353b793cc3ade33a84fae38cbc5ee9dd6a2ee8cc5bf84ba


Bot(s) for this bug's original alert(s):

chromium-rel-mac11-air

📍 Pinpoint job started.
https://pinpoint-dot-chromeperf.appspot.com/job/12c9c0c8240000

😿 Pinpoint job stopped with an error.
https://pinpoint-dot-chromeperf.appspot.com/job/12c9c0c8240000

Buildbucket says the build completed successfully, but Pinpoint can't find the isolate hash.

[Edited, adding the commit hash and explanation].

Bisection indicates this commit in V8:

commit 600641338d160ce2dc82d3cdec0f414946409b81
Author: Dan Elphick <delphick@chromium.org>
Date:   Tue May 8 13:50:28 2018 +0100

    Reland "[heap] Mark RO_SPACE as read-only after deserialization"
    
    This is a reland of 40f1aaf330dc37f12bd5015b7d11dcbebfb55c59
    
    Put back padding clearing into the SerializeObject method but only when
    the String is not in RO_SPACE. For RO_SPACE strings, if required
    iterate over the space before serialization clearing the strings.
    
    Original change's description:
    > [heap] Mark RO_SPACE as read-only after deserialization
    >
    > Adds MarkAsReadOnly and MarkAsReadWrite to ReadOnlySpace. The latter
    > is only usable with ReadOnlySpace::WritableScope to avoid the space
    > being left writable). MarkAsReadOnly updates the high water mark and
    > makes several previously mutating methods into no-ops.
    >
    > Moves some writes to immutable objects out of the bootstrapper to
    > setup-heap-internal so they don't write to a read-only page.
    >
    > Also avoid writing hashes to strings that already have the value set as
    > that invariably means writing to the "0" and "1" constant strings in
    > RO_SPACE.
    >
    > Before serialization, it makes RO_SPACE writable again so that any
    > padding can be cleared before writing it.
    >
    > Bug: v8:7464
    > Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
    > Change-Id: I22edc20dba7dde8943991a8fcaf87244af4490a3
    > Reviewed-on: https://chromium-review.googlesource.com/1014128
    > Commit-Queue: Dan Elphick <delphick@chromium.org>
    > Reviewed-by: Hannes Payer <hpayer@chromium.org>
    > Reviewed-by: Yang Guo <yangguo@chromium.org>
    > Cr-Commit-Position: refs/heads/master@{#52943}
    
    Bug: v8:7464
    Change-Id: Ia8386c4ff5f5df3207f584caf7a9b1ff1e405f25
    Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
    Reviewed-on: https://chromium-review.googlesource.com/1042145
    Reviewed-by: Hannes Payer <hpayer@chromium.org>
    Reviewed-by: Jakob Gruber <jgruber@chromium.org>
    Commit-Queue: Dan Elphick <delphick@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#53087}

This appears to change the way string externalisation works. Comparison of perf report output indicates new functions are getting called:

Trace after:
   6.98%  chrome           chrome                       [.] _ZN5blink7Element16AttributeChangedERKNS0_27AttributeModificationParamsE
   6.70%  chrome           chrome                       [.] _ZN5blink9V8Element26setAttributeMethodCallbackERKN2v820FunctionCallbackInfoINS1_5ValueEEE
   6.46%  chrome           chrome                       [.] _ZN5blink7Element12setAttributeERKN3WTF12AtomicStringES4_RNS_14ExceptionStateE
   6.14%  chrome           chrome                       [.] _ZN5blink16V8StringResourceILNS_20V8StringResourceModeE0EE11PrepareFastEv
   5.13%  chrome           chrome                       [.] _ZNK3WTF9HashTableIN5blink13QualifiedNameENS_12KeyValuePairIS2_NS1_20AnimatedPropertyTypeEEENS_24KeyValuePairKeyExtractorENS1_17QualifiedNameHashENS_18Ha
   5.11%  chrome           chrome                       [.] _ZN5blink23V8StringToWebCoreStringIN3WTF12AtomicStringEEET_N2v85LocalINS4_6StringEEENS_12ExternalModeE
   4.33%  chrome           chrome                       [.] _ZN3WTF9HashTableIPNS_10StringImplES2_NS_17IdentityExtractorENS_10StringHashENS_10HashTraitsIS2_EES6_NS_18PartitionAllocatorEE20FullLookupForWritingINS_2
   2.54%  chrome           chrome                       [.] _ZNK2v86String12WriteOneByteEPhiii
   2.39%  chrome           chrome                       [.] _ZN2v88internal6String11WriteToFlatIhEEvPS1_PT_ii
   2.38%  chrome           chrome                       [.] _ZN3WTF17AtomicStringTable8InstanceEv
   2.31%  chrome           chrome                       [.] _ZNK5blink26AttributeCollectionGenericIKNS_14AttributeArrayES2_E9FindIndexERKN3WTF12AtomicStringE
   2.23%  chrome           chrome                       [.] _ZN5blink11HTMLElement24TriggersForAttributeNameERKNS_13QualifiedNameE
   2.15%  chrome           chrome                       [.] _ZN5blink7Element18DidModifyAttributeERKNS_13QualifiedNameERKN3WTF12AtomicStringES7_
   2.05%  chrome           chrome                       [.] _ZNK4base18ThreadLocalStorage4Slot3GetEv
   1.90%  chrome           chrome                       [.] _ZN3WTF12AtomicStringC1EPKhj
   1.86%  chrome           chrome                       [.] _ZNK3WTF12AtomicString10LowerASCIIEv
   1.77%  chrome           chrome                       [.] _ZN5blink7Element19WillModifyAttributeERKNS_13QualifiedNameERKN3WTF12AtomicStringES7_
   1.75%  chrome           chrome                       [.] _ZN3WTF10StringImpl10LowerASCIIEv
   1.57%  chrome           chrome                       [.] _ZN5blink8Document11IsValidNameERKN3WTF6StringE
   1.51%  chrome           chrome                       [.] _ZN5blink13ContainerNode35InvalidateNodeListCachesInAncestorsEPKNS_13QualifiedNameEPNS_7ElementEPKNS0_14ChildrenChangeE
   1.49%  chrome           chrome                       [.] _ZN5blink11HTMLElement16AttributeChangedERKNS_7Element27AttributeModificationParamsE
   1.34%  chrome           chrome                       [.] _ZN5blink12StringTraitsIN3WTF12AtomicStringEE12FromV8StringINS_20V8StringOneByteTraitEEES2_N2v85LocalINS6_6StringEEEi
   1.32%  chrome           chrome                       [.] _ZN3WTF9HashTableIPNS_10StringImplES2_NS_17IdentityExtractorENS_10StringHashENS_10HashTraitsIS2_EES6_NS_18PartitionAllocatorEE21InsertPassingHashCodeINS_
   1.23%  chrome           chrome                       [.] _ZN5blink18StringResourceBase15GetAtomicStringEv
   1.18%  chrome           chrome                       [.] _ZN2v86String15CanMakeExternalEv
   1.10%  chrome           chrome                       [.] _ZN5blink11HTMLElement14ParseAttributeERKNS_7Element27AttributeModificationParamsE
   1.06%  chrome           chrome                       [.] _ZNK5blink7Element20SynchronizeAttributeERKN3WTF12AtomicStringE
   0.93%  chrome           chrome                       [.] _ZN5blink13QualifiedNameD1Ev
   0.91%  chrome           libpthread-2.19.so           [.] pthread_getspecific
   0.83%  chrome           chrome                       [.] _ZN2v88internal6String7FlattenENS0_6HandleIS1_EENS0_13PretenureFlagE
   0.77%  chrome           chrome                       [.] _ZNK5blink8Document30ShouldInvalidateNodeListCachesEPKNS_13QualifiedNameE
   0.66%  chrome           chrome                       [.] FcCompareValueList
   0.60%  chrome           libc-2.19.so                 [.] __memcmp_sse4_1
   0.59%  chrome           chrome                       [.] FcCompareFamily
   0.53%  chrome           chrome                       [.] _ZN3WTF5EqualEPKNS_10StringImplEPKhj
   0.39%  chrome           chrome                       [.] _ZNK2v86String19ContainsOnlyOneByteEv

Trace before:
  11.59%  chrome           chrome                         [.] _ZN5blink9V8Element26setAttributeMethodCallbackERKN2v820FunctionCallbackInfoINS1_5ValueEEE
  10.68%  chrome           chrome                         [.] _ZN5blink7Element12setAttributeERKN3WTF12AtomicStringES4_RNS_14ExceptionStateE
   9.68%  chrome           chrome                         [.] _ZN5blink7Element16AttributeChangedERKNS0_27AttributeModificationParamsE
   4.99%  chrome           chrome                         [.] _ZN5blink16V8StringResourceILNS_20V8StringResourceModeE0EE11PrepareFastEv
   4.44%  chrome           chrome                         [.] _ZN5blink23V8StringToWebCoreStringIN3WTF12AtomicStringEEET_N2v85LocalINS4_6StringEEENS_12ExternalModeE
   4.11%  chrome           chrome                         [.] _ZNK3WTF9HashTableIN5blink13QualifiedNameENS_12KeyValuePairIS2_NS1_20AnimatedPropertyTypeEEENS_24KeyValuePairKeyExtractorENS1_17QualifiedNameHashENS_18
   3.60%  chrome           chrome                         [.] _ZN5blink7Element18DidModifyAttributeERKNS_13QualifiedNameERKN3WTF12AtomicStringES7_
   3.32%  chrome           chrome                         [.] _ZNK3WTF12AtomicString10LowerASCIIEv
   3.05%  chrome           chrome                         [.] _ZN5blink11HTMLElement16AttributeChangedERKNS_7Element27AttributeModificationParamsE
   2.67%  chrome           chrome                         [.] _ZN3WTF10StringImpl10LowerASCIIEv
   2.63%  chrome           chrome                         [.] _ZN5blink8Document11IsValidNameERKN3WTF6StringE
   2.46%  chrome           chrome                         [.] _ZNK5blink26AttributeCollectionGenericIKNS_14AttributeArrayES2_E9FindIndexERKN3WTF12AtomicStringE
   2.38%  chrome           chrome                         [.] _ZN5blink7Element19WillModifyAttributeERKNS_13QualifiedNameERKN3WTF12AtomicStringES7_
   2.21%  chrome           chrome                         [.] _ZN5blink13ContainerNode35InvalidateNodeListCachesInAncestorsEPKNS_13QualifiedNameEPNS_7ElementEPKNS0_14ChildrenChangeE
   2.03%  chrome           chrome                         [.] _ZN5blink11HTMLElement24TriggersForAttributeNameERKNS_13QualifiedNameE
   1.94%  chrome           chrome                         [.] _ZN5blink18StringResourceBase15GetAtomicStringEv
   1.57%  chrome           chrome                         [.] _ZN5blink11HTMLElement14ParseAttributeERKNS_7Element27AttributeModificationParamsE
   1.39%  chrome           chrome                         [.] _ZN5blink13QualifiedNameD1Ev
   1.24%  chrome           chrome                         [.] _ZNK5blink7Element20SynchronizeAttributeERKN3WTF12AtomicStringE
   1.17%  chrome           chrome                         [.] _ZNK5blink8Document30ShouldInvalidateNodeListCachesEPKNS_13QualifiedNameE

Downgrading P2s that haven't been modified in more than 6 months, which have no component or owner.