This is a reland of a9a50dc9

Buildbot crashes are fixed by a274fc65.

Original change's description:
> [heap] Temporarily disable concurrent marking.
> 
> The 6.3 branch has a data race that is fixed in 6.4 but the fix is too
> large for back merging.
> 
> This CL will be back-merged to 6.3 after getting Canary coverage.
> 
> Concurrent marking will be re-enabled afterwards.
> 
> Bug: chromium:774644
> Change-Id: I4112da0e133a637cc4fb52dee2e4c165cdc74f1f
> Reviewed-on: https://chromium-review.googlesource.com/749811
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49080}

Bug: chromium:774644
Change-Id: Idf5d179eca25a1481c70c6ca3bccde4869deb544
Reviewed-on: https://chromium-review.googlesource.com/751271Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49090}