Adding some nullptr checks around calls to Canvas2DLayerBridge::getOrCreateSurface

This change is to make canvas more resilient to memory allocation
failures and GPU context losses by dropping content instead of
crashing the render process.

BUG=569971

Review URL: https://codereview.chromium.org/1529903002

Cr-Commit-Position: refs/heads/master@{#365331}
1 file changed