Use MessageLoopCurrent::IsSet in /components/policy

As stated in message_loop_current.h, |MessageLoopCurrent::IsSet()|
should be preferred to checking the return value of
|MessageLoopCurrent::Get()|.

Bug: 825327
Change-Id: I90bf74dd9929eeb946e8e2e3ee1ac2e9f093a938
Reviewed-on: https://chromium-review.googlesource.com/1025770
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553086}
1 file changed