Don't racily read a boolean flag - it makes TSAN unhappy.

A flag was set under mutex protection, but could be racily read.
A comment indicating that using the stale state would be safe is
probably correct, but racy data use makes TSAN a sad panda.

BUG=418807

Review URL: https://codereview.chromium.org/618083002

git-svn-id: svn://svn.chromium.org/blink/trunk@183150 bbb929c8-8fbe-4397-9dbb-9b2b20218538