MSan: disallow uninitialized data in GLES2Implementation::BufferDataHelper.

MSan: disallow uninitialized data in GLES2Implementation::BufferDataHelper.

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

Committed: https://crrev.com/c0d26f39a0d4a98f271e13c46d143bac1eb0e9e0
Cr-Commit-Position: refs/heads/master@{#314098}

[earthdok, 2015-01-30 18:54:53]

please take a look

[Zhenyao Mo, 2015-01-30 20:07:51]

On 2015/01/30 18:54:53, earthdok wrote:
> please take a look

LGTM

[earthdok, 2015-01-30 20:08:12]

The CQ bit was checked by earthdok@chromium.org

[commit-bot: I haz the power, 2015-01-30 20:08:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/867903004/1

[commit-bot: I haz the power, 2015-01-30 21:20:23]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-01-30 21:20:24]

Try jobs failed on following builders:
  mac_chromium_rel_ng on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[earthdok, 2015-02-01 23:02:19]

The CQ bit was checked by earthdok@chromium.org

[commit-bot: I haz the power, 2015-02-01 23:03:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/867903004/1

[commit-bot: I haz the power, 2015-02-01 23:42:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-01 23:44:03]

Patchset 1 (id:??) landed as
https://crrev.com/c0d26f39a0d4a98f271e13c46d143bac1eb0e9e0
Cr-Commit-Position: refs/heads/master@{#314098}