Wrap the conditional in CHECK with UNLIKELY since it is.

This matches what was done in RELEASE_ASSERT in blink. No perf
bots noticed this being missing (it does nothing on windows which
is where perf bots noticed anything) but it won't hurt right.

R=dcheng@chromium.org
BUG=672699

Review-Url: https://codereview.chromium.org/2589943002
Cr-Commit-Position: refs/heads/master@{#439860}
1 file changed