Hold context lock while retrieving GrContextThreadSafeProxy
Hold context lock while retrieving GrContextThreadSafeProxy

We need to hold the context lock while retrieving
GrContextThreadSafeProxy. This adds the lock logic to the constructor
of GpuImageDecodeController.

CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1900983002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388399}

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