Check display list canvas fall back reason unknown

The FallbackReasonUnknown bin is still non-zero. This CL changes DCHECK
to CHECK, such that when Unknown happens, browser crashes. This will hit
canary first, once we have crash report with stack trace, we can fix it
right away.

BUG=585578

Review-Url: https://codereview.chromium.org/2125153002
Cr-Commit-Position: refs/heads/master@{#404040}
1 file changed