Insist on a Blink GC on RenderViewImpl unit test teardowns.
Insist on a Blink GC on RenderViewImpl unit test teardowns.

Without it, flaky LSan reports are a risk, depending on GC timing and
on tests allowing posted GC tasks to be processed before many of them
abruptly clear posted messages as part of their operation.

R=haraken
TBR=jochen
BUG=

Review URL: https://codereview.chromium.org/1621823004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#371169}

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