See 

It's not coincident with a volume shift, is historically large in the 90 day time frame, and is coincident with a Chrome release (50.0.2661.102).  I tried to check to see if it happened on Beta, but we don't appear to have ever released that version on beta.  I looked at net/ commits between the last m50 release on beta and this one, and there were only two:

    Disable CertVerifyProcTest TestKnownRoot and PublicKeyHashes
    QUIC - Fix a type casting bug in quic stream sequencer buffer.

(full details below) and neither one seemed likely to have caused a problem like this.  

Having said that, it's still a very small proportion of the entries in the histogram, so probably not a high priority.  But signs point to a real change in Chrome.

commit e23f722cebb9f8bb1a722679a184faf377e34ea9
Author: Jackie Quinn <jyquinn@chromium.org>
Date:   Tue May 10 17:05:43 2016 -0700

    Disable CertVerifyProcTest TestKnownRoot and PublicKeyHashes
    
    Comment in TestKnownRoot indicates that it and PublicKeyHashes will
    fail on May 9, 2016, and they did. Disabling for now.
    
    BUG= 610546 
    TBR=agl@chromium.org
    NOTRY=true
    NOPRESUBMIT=true
    
    Review-Url: https://codereview.chromium.org/1959083003
    Cr-Commit-Position: refs/heads/master@{#392503}
    (cherry picked from commit f8907f402e6141d88721bb4b4307c8fe8f283c5e)
    
    Review URL: https://codereview.chromium.org/1963403002 .
    
    Cr-Commit-Position: refs/branch-heads/2661@{#679}
    Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}

commit d2116bf269549309bf6272cab02e55bad406227f
Author: ckrasic <ckrasic@chromium.org>
Date:   Mon Apr 18 14:25:14 2016 -0700

    QUIC - Fix a type casting bug in quic stream sequencer buffer.
    
    This change is manually created against branch 2661 (M50) for post
    release merge.
    
    Without this change, a QUIC frame with offset higher than 2^32 might corrupt StreamSequencerBuffer's missing frame recording logic in a 32-bit machine.
    
    Merge internal change: 117245736
    
    BUG= 603656 
    NOTRY=true
    NOPRESUBMIT=true
    
    Review URL: https://codereview.chromium.org/1893803002
    
    Cr-Commit-Position: refs/branch-heads/2661@{#601}
    Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}