Consider that |gpu_channel_| may have been lost.
Consider that |gpu_channel_| may have been lost.

This continues the investigation of bug 495185, which was made difficult
by crash dumps not including this information.

BUG=495185

Review URL: https://codereview.chromium.org/1277653002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342184}

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