VideoCaptureBufferPool: remove unnecessary |data_| pointers after GpuMemoryBuffer API change

VideoCaptureBufferPool: remove unnecessary |data_| pointers after GpuMemoryBuffer API change

GpuMemoryBufferBufferHandle is a VideoCaputerBufferPool::BufferHandle
created from VCBP::GpuMemoryBufferTracker::GpuMemoryBufferTracker().
Previously, the former inherited the data pointer from the latter,
but with the recent change in GpuMemoryBuffer API [1], this is not
needed anymore. This CL removes the member variable and adapts
DCHECK()s in the vicinity.

BUG=542225
TEST= All unittests, content_browsertests and browser_tests passing.

[1] http://crrev.com/1401183003, relanded as (1412223009).

Committed: https://crrev.com/681f844172cf1b945b345f345f1878c6dca02339
Cr-Commit-Position: refs/heads/master@{#356770}

[mcasas, 2015-10-26 23:23:59]

mcasas@chromium.org changed reviewers:
+ emircan@chromium.org, miu@chromium.org

[mcasas, 2015-10-26 23:23:59]

miu@ PTAL (corollary follow up of a previous CL, see bug).

(ermican@ FYI/PTAL)

[mcasas, 2015-10-27 17:36:32]

Patchset #1 (id:1) has been deleted

[mcasas, 2015-10-27 17:36:40]

Patchset #1 (id:20001) has been deleted

[emircan, 2015-10-27 21:31:47]

lgtm

[emircan, 2015-10-27 21:31:49]

lgtm

[miu, 2015-10-29 01:03:42]

lgtm

[mcasas, 2015-10-29 03:57:45]

The CQ bit was checked by mcasas@chromium.org

[commit-bot: I haz the power, 2015-10-29 03:58:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1422123002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1422123002/40001

[commit-bot: I haz the power, 2015-10-29 05:18:15]

Committed patchset #1 (id:40001)

[commit-bot: I haz the power, 2015-10-29 05:18:52]

Patchset 1 (id:??) landed as
https://crrev.com/681f844172cf1b945b345f345f1878c6dca02339
Cr-Commit-Position: refs/heads/master@{#356770}