This scope is used to control microtasks execution when MicrotasksPolicy::kScoped is engaged.

Attempt #2. First one was reverted due to chromium breakage: SetAutorunMicrotasks(false) was broken.

BUG=chromium:585949
LOG=Y
TEST=ScopedMicrotasks

Review URL: https://codereview.chromium.org/1741893003

Cr-Commit-Position: refs/heads/master@{#34504}