Remove const WebMediaPlayer::didLoadingProgress() implementations.

Remove const WebMediaPlayer::didLoadingProgress() implementations.

This is the third of three changes; the others are:
  https://codereview.chromium.org/275303002 (chromium prep)
  https://codereview.chromium.org/272223004 (blink side)

BUG=360251
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270520

[scherkus (not reviewing), 2014-05-13 00:12:54]

lgtm

[sandersd (OOO until July 31), 2014-05-14 18:09:40]

The CQ bit was checked by sandersd@chromium.org

[commit-bot: I haz the power, 2014-05-14 18:10:20]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sandersd@chromium.org/284723003/1

[commit-bot: I haz the power, 2014-05-14 21:26:40]

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:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)
  linux_chromium_chromeos_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)
  linux_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel/bu...)

[commit-bot: I haz the power, 2014-05-14 21:40:11]

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

[commit-bot: I haz the power, 2014-05-14 21:40:11]

Try jobs failed on following builders:
  linux_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel/bu...)

[sandersd (OOO until July 31), 2014-05-14 21:53:23]

The CQ bit was checked by sandersd@chromium.org

[commit-bot: I haz the power, 2014-05-14 21:56:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sandersd@chromium.org/284723003/1

[commit-bot: I haz the power, 2014-05-14 23:02:48]

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_chromium_chromeos_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)
  linux_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel/bu...)

[commit-bot: I haz the power, 2014-05-15 00:02:37]

Change committed as 270520