Fix WebGL context restoration logic. The retry mechanism was broken when the ownership of the WebGraphicsContext3D was transferred to the DrawingBuffer.

The Telemetry based context loss test is being made more of a stress test to verify this fix. Will also attempt to make the WebGL conformance tests more stressful to catch the underlying race condition more reliably.

BUG=365904

Review URL: https://codereview.chromium.org/254453002

git-svn-id: svn://svn.chromium.org/blink/trunk@172465 bbb929c8-8fbe-4397-9dbb-9b2b20218538