GitLab

In a previous CL: https://chromium-review.googlesource.com/c/579690 where
we refactor the Canvas2DLayerBridge, we did not null check before passing
calling the ReleaseMailboxImageResource.

This CL does the null check.

Bug: 751445
Change-Id: I86f59e8176a44ef53d591a560a4c0c4b6cad2676
Reviewed-on: https://chromium-review.googlesource.com/597226
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491551}