Change ASSERT to CHECK in DrawingBuffer::reset for investigation

Changing this assert to a CHECK which will re-introduce crashes
related to bug 542857, but will result in crash reports that
will allow us to understand how the DrawingBuffer got into
a bad state.

CL to be reverted before next branch point.

BUG=542857
TBR=kbr@chromium.org

Review-Url: https://codereview.chromium.org/2126603004
Cr-Commit-Position: refs/heads/master@{#403824}
1 file changed