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This patch introduces a WebGL test that ensures we don't hit an
assertion failure in DrawingBuffer when the context is lost in a
background tab.

Depends on the Blink fix from https://codereview.chromium.org/556303002.

BUG=411372

Review URL: https://codereview.chromium.org/560023002

Cr-Commit-Position: refs/heads/master@{#294417}