Change blink::WebMediaPlayer::buffered() to return by-value

Change blink::WebMediaPlayer::buffered() to return by-value

This removes the need for implementations to hand out references to mutable data.

Chromium change is https://codereview.chromium.org/282073003/.

BUG=360254
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174393

[darin (slow to review), 2014-05-16 00:02:27]

LGTM

[sandersd (OOO until July 31), 2014-05-16 18:05:40]

The CQ bit was checked by sandersd@chromium.org

[commit-bot: I haz the power, 2014-05-16 18:07:12]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sandersd@chromium.org/284143002/1

[commit-bot: I haz the power, 2014-05-16 19:35:01]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  linux_blink_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_dbg/builds/7641)
  mac_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_rel/builds/6954)

[commit-bot: I haz the power, 2014-05-17 17:45:25]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  linux_blink_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_dbg/builds/7838)

[commit-bot: I haz the power, 2014-05-17 23:45:49]

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

[commit-bot: I haz the power, 2014-05-17 23:45:50]

Try jobs failed on following builders:
  linux_blink_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_dbg/builds/7838)

[sandersd (OOO until July 31), 2014-05-20 18:29:58]

The CQ bit was checked by sandersd@chromium.org

[commit-bot: I haz the power, 2014-05-20 18:30:12]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sandersd@chromium.org/284143002/1

[commit-bot: I haz the power, 2014-05-20 18:30:43]

Change committed as 174393