Remove software paint FrameSubscriber from tab capture.

Remove software paint FrameSubscriber from tab capture.

All tab capture platforms have supported capture via event-driven
compositor updates for quite a while now.  The "notification of backing
store change" code path never triggers, except in certain edge cases
where extra frame captures will be started.  The extra frame captures
are bad for the user experience, since the extra frames reduce video
playback smoothness.  The extra frame captures were also throwing-off
the "out-of-order frame delivery" logic, causing wanted frames to be
dropped.

BUG=464475
Committed: https://crrev.com/365c4b25fb7cfcebf166ca7b24d11561731f5b80
Cr-Commit-Position: refs/heads/master@{#319549}

[miu, 2015-03-06 23:13:26]

miu@chromium.org changed reviewers:
+ hubbe@chromium.org

[miu, 2015-03-06 23:13:27]

hubbe: PTAL.

[hubbe, 2015-03-06 23:39:09]

lgtm

[miu, 2015-03-07 00:18:29]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2015-03-07 00:19:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/989683004/1

[commit-bot: I haz the power, 2015-03-07 01:13:49]

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

[commit-bot: I haz the power, 2015-03-07 01:13:50]

Try jobs failed on following builders:
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_TIMED_OUT, no build
URL)
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_TIMED_OUT, no build URL)
Timed out jobs are not retried to avoid causing additional load on the builders
with large pending queues.

[miu, 2015-03-07 01:51:49]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2015-03-07 01:53:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/989683004/1

[commit-bot: I haz the power, 2015-03-07 03:14:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-07 03:15:50]

Patchset 1 (id:??) landed as
https://crrev.com/365c4b25fb7cfcebf166ca7b24d11561731f5b80
Cr-Commit-Position: refs/heads/master@{#319549}