Example site to repro repeated main-thread-GC issue with:
https://kunstmusik.github.io/audioworklet-gc

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/ca3e146ecbaad449f6cdc8539d9f0c7920d2903d

commit ca3e146ecbaad449f6cdc8539d9f0c7920d2903d
Author: Wez <wez@chromium.org>
Date: Fri May 04 11:15:18 2018

Rename PromotedSpaceSizeOfObjects() to OldGenerationSizeOfObjects().

The Promoted* prefix was used to refer both to the total number of old
generation objects, and to the delta of objects moved from the new to
old generations.

PromotedTotalSize() is also renamed, to reflect the actual calculation
it performs

Bug:  chromium:837583 
Change-Id: Id27a0661618257ef64eb469a83bb49c0e8ce6923
Reviewed-on: https://chromium-review.googlesource.com/1042314
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52986}
[modify] https://crrev.com/ca3e146ecbaad449f6cdc8539d9f0c7920d2903d/src/heap/heap.cc
[modify] https://crrev.com/ca3e146ecbaad449f6cdc8539d9f0c7920d2903d/src/heap/heap.h
[modify] https://crrev.com/ca3e146ecbaad449f6cdc8539d9f0c7920d2903d/src/heap/incremental-marking.cc
[modify] https://crrev.com/ca3e146ecbaad449f6cdc8539d9f0c7920d2903d/test/cctest/heap/test-heap.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/aca333123dad7a7e9c65a43c9e27ab610435b0e3

commit aca333123dad7a7e9c65a43c9e27ab610435b0e3
Author: Wez <wez@chromium.org>
Date: Tue May 15 21:04:42 2018

[heap] Replace retained_size() with ExternalBackingStoreBytes().

ArrayBuffer memory allocated off-heap was previously tracked by a test-
only retained_size() field on each LocalArrayBufferTracker.

Changes in off-heap ArrayBuffer memory usage are now reported to the
Space with which the ArrayBuffer is associated, so that the value is
cheaply available to include in e.g. GC limit calculations, via a new
getter, ExternalBackingStoreBytes().

Changes to external ArrayBuffer backing-store allocations are tracked in
an AtomicNumber associated with each Space, to allow for ArrayBuffers
being concurrently moved or freed from multiple Pages in the same Space
during sweeps & compactions.

Bug:  chromium:837583 
Change-Id: I8b1b6addd5cd05533d8da55ca813e134bc36e181
Reviewed-on: https://chromium-review.googlesource.com/1052347
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53192}
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/src/heap/array-buffer-collector.cc
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/src/heap/array-buffer-tracker-inl.h
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/src/heap/array-buffer-tracker.cc
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/src/heap/array-buffer-tracker.h
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/src/heap/spaces.cc
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/src/heap/spaces.h
[modify] https://crrev.com/aca333123dad7a7e9c65a43c9e27ab610435b0e3/test/cctest/heap/test-array-buffer-tracker.cc