Add trace event for DrawingBuffer::prepareMailbox in blink category.

(Attempted to put this in the "gpu" category but ran into
http://crbug.com/630030 .)

This will help in the creation of TBMv2 WebGL benchmarks.

BUG=catapult:#2446

Review-Url: https://codereview.chromium.org/2163373005
Cr-Commit-Position: refs/heads/master@{#406737}
1 file changed