This is to catch remaining instances or main thread's RCS accessed from other
threads. It could have a small negative impact on performance with RCS enabled.
We are going to revert this patch within a week.

BUG=chromium:760649

Change-Id: I437bf7206829c813c0090552c031199840f4baf4
Reviewed-on: https://chromium-review.googlesource.com/728398Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48761}