Is there a straightforward layout-test-ish thing we could use to verify that heap compaction is working as intended for all the affected arenas?  Or perhaps a unit-test would be easier?

There already is (HeapCompactTest), but as the feature isn't observable / controllable, we rely on verifying functional behavior staying the same across those compaction runs rather than try to guess&observe if a object moved or not (and somehow test for that.)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ec3056b0028de68485f6e5c4b6d57f225d334605

commit ec3056b0028de68485f6e5c4b6d57f225d334605
Author: sigbjornf <sigbjornf@opera.com>
Date: Fri May 26 08:24:26 2017

Enable heap compaction on all 'container' arenas.

Reconcile indexing for compactable arenas to always be wrt
the arena index -- it was miscued on setting, using an
offset instead, thereby preventing vector[1-4] arenas from
being considered for compaction.

R=haraken
BUG= 726335 

Review-Url: https://codereview.chromium.org/2908463002
Cr-Commit-Position: refs/heads/master@{#474966}

[modify] https://crrev.com/ec3056b0028de68485f6e5c4b6d57f225d334605/third_party/WebKit/Source/platform/heap/HeapCompact.cpp
[modify] https://crrev.com/ec3056b0028de68485f6e5c4b6d57f225d334605/third_party/WebKit/Source/platform/heap/HeapCompact.h
[modify] https://crrev.com/ec3056b0028de68485f6e5c4b6d57f225d334605/third_party/WebKit/Source/platform/heap/HeapCompactTest.cpp
[modify] https://crrev.com/ec3056b0028de68485f6e5c4b6d57f225d334605/third_party/WebKit/Source/platform/heap/HeapPage.cpp

I'd expect some improvements to renderer process sizes, so something to perhaps check status of once rolled out.

Thanks again for reporting this.