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MSan doesn't undestand shared memory, so a buffer that has been tainted with
uninitialized values would still be considered uninitialized even after a write
on the other end (the GPU). This results in confusing uninit reports in the
renderer after readback. Doing an explicit check early should make those reports
less confusing and less flaky. We might even find some real bugs!

BUG=445745
R=zmo@chromium.org

Review URL: https://codereview.chromium.org/867903004

Cr-Commit-Position: refs/heads/master@{#314098}