Annotate CaptureDumpPixels for MemorySanitizer.

Annotate CaptureDumpPixels for MemorySanitizer.

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

Committed: https://crrev.com/10538620e806691c97c63ba33d21900104b31eea
Cr-Commit-Position: refs/heads/master@{#295490}

[earthdok, 2014-09-18 17:46:23]

The CQ bit was checked by earthdok@chromium.org

[piman, 2014-09-18 17:47:32]

lgtm

[commit-bot: I haz the power, 2014-09-18 17:48:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/583703002/1

[commit-bot: I haz the power, 2014-09-18 17:49:46]

Committed patchset #1 (id:1) as e8647a26799f83d8a211a0b673c9885c8b5b3342

[commit-bot: I haz the power, 2014-09-18 17:50:30]

Patchset 1 (id:??) landed as
https://crrev.com/10538620e806691c97c63ba33d21900104b31eea
Cr-Commit-Position: refs/heads/master@{#295490}