media: Allow WMPI to send playing started notification while playing

media: Allow WMPI to send playing started notification while playing

Previously MediaWebContentsObserver DCHECKed that an active player
was never added twice. To avoid complicated logic in WMPI to keep track
of when it's okay to send a playing notification to
MediaWebContentsObserver, now it's allowed to send it more than once.

BUG=595903, 595970
Review URL: https://codereview.chromium.org/1810213002

Cr-Commit-Position: refs/heads/master@{#382098}
(cherry picked from commit 73fd2eb18f1ed48a1047d737a9db170608ac6f47)

Committed: https://chromium.googlesource.com/chromium/src/+/217c5e96c255d046fce4a1743b560e7ee5625a82

[watk, 2016-03-21 18:10:49]

Description was changed from

==========
media: Allow WMPI to send playing started notification while playing

Previously MediaWebContentsObserver DCHECKed that an active player
was never added twice. To avoid complicated logic in WMPI to keep track
of when it's okay to send a playing notification to
MediaWebContentsObserver, now it's allowed to send it more than once.

BUG=595903, 595970

Review URL: https://codereview.chromium.org/1810213002

Cr-Commit-Position: refs/heads/master@{#382098}
(cherry picked from commit 73fd2eb18f1ed48a1047d737a9db170608ac6f47)
==========

to

==========
media: Allow WMPI to send playing started notification while playing

Previously MediaWebContentsObserver DCHECKed that an active player
was never added twice. To avoid complicated logic in WMPI to keep track
of when it's okay to send a playing notification to
MediaWebContentsObserver, now it's allowed to send it more than once.

BUG=595903, 595970

Review URL: https://codereview.chromium.org/1810213002

Cr-Commit-Position: refs/heads/master@{#382098}
(cherry picked from commit 73fd2eb18f1ed48a1047d737a9db170608ac6f47)

Committed:
https://chromium.googlesource.com/chromium/src/+/217c5e96c255d046fce4a1743b56...
==========

[watk, 2016-03-21 18:10:51]

Committed patchset #1 (id:1) manually as
217c5e96c255d046fce4a1743b560e7ee5625a82.