The mock histogram functions cannot be cleared and can be called on
isolate tear down if incremental marking is in progress.

Bug: chromium:850508
Tbr: mlippautz@chromium.org
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I99e52aaa81c863f71e195aeed691b37da9e71da6
Reviewed-on: https://chromium-review.googlesource.com/1093073Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53616}