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: refs/heads/master@{#371169}
1 file changed