Fix HTMLMediaElement "direction of playback" behavior.

Fix HTMLMediaElement "direction of playback" behavior.

This change cleans up the code a little to make it easier to see that
the "direction of playback" is being used and corrects the definition of
"forward" to include playbackRate == 0. New test cases were added to
expose the change in behavior.

BUG=385343
TESTS=LayoutTests/media/media-ended.html LayoutTests/media/video-seek-to-duration-with-playbackrate-zero.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176264

[adamk, 2014-06-16 23:15:50]

lgtm

[acolwell GONE FROM CHROMIUM, 2014-06-16 23:17:25]

The CQ bit was checked by acolwell@chromium.org

[commit-bot: I haz the power, 2014-06-16 23:17:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/acolwell@chromium.org/331063006/1

[commit-bot: I haz the power, 2014-06-17 01:08:57]

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

[commit-bot: I haz the power, 2014-06-17 01:08:57]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/12192)

[acolwell GONE FROM CHROMIUM, 2014-06-17 01:21:45]

The CQ bit was checked by acolwell@chromium.org

[commit-bot: I haz the power, 2014-06-17 01:22:01]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/acolwell@chromium.org/331063006/1

[commit-bot: I haz the power, 2014-06-17 02:00:18]

Change committed as 176264