Do not flush main thread tasks in ~ScopedTaskEnvironment.

Flushing main thread tasks in ~ScopedTaskEnvironment prevents
MessageLoop from being replaced with ScopedTaskEnvironment in v8
tests that don't expect tasks to run without an explicit call to
RunLoop::Run/RunUntilIdle.

BUG=708584

Review-Url: https://codereview.chromium.org/2876583002
Cr-Commit-Position: refs/heads/master@{#470642}
2 files changed