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The snapshot pixels come from shared memory, which is initialized in the GPU
process. MSan doesn't know what goes on in other processes, so we need to
explicitly mark the pixels as initialized to silence MSan warnings.

BUG=364724
TBR=piman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/583703002

Cr-Commit-Position: refs/heads/master@{#295490}