Ensure that hasPendingActivity() is false for stopped media elements

Ensure that hasPendingActivity() is false for stopped media elements

Where stopped means that ActiveDOMObject::stop() has been called.

All the conditions in hasPendingActivity() are now false after stop():

 - m_shouldDelayLoadEvent is set to false by userCancelledLoad() ->
   setShouldDelayLoadEvent(false).

 - m_networkState is either NETWORK_EMPTY or NETWORK_IDLE after
   userCancelledLoad().

 - couldPlayEnoughData() returns false because m_paused is now set to
   true by stop().

 - m_seeking is now set to false by stop().

 - m_mediaSource is set to null by userCancelledLoad() ->
   closeMediaSource().

 - m_asyncEventQueue->hasPendingEvents() is false because stop() calls
   m_asyncEventQueue->close().

TEST=LayoutTests/media/gc-pending-event-inactive-document.html

This test reliably hits the assert that revealed this problem:
https://codereview.chromium.org/584633004

BUG=400659
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182332

[philipj_slow, 2014-09-19 13:45:39]

philipj@opera.com changed reviewers:
+ fs@opera.com

[philipj_slow, 2014-09-19 13:45:39]

PTAL. Instead of https://codereview.chromium.org/585903002/

[fs, 2014-09-19 13:50:00]

LGTM

Description nit:

"couldPlayEnoughData() returns false because m_paused is now set to false by
stop()."

s/m_paused is now set to false/m_paused is now set to true/

[philipj_slow, 2014-09-19 13:53:20]

On 2014/09/19 13:50:00, fs wrote:
> LGTM
> 
> Description nit:
> 
> "couldPlayEnoughData() returns false because m_paused is now set to false by
> stop()."
> 
> s/m_paused is now set to false/m_paused is now set to true/

Fixed, thanks!

[philipj_slow, 2014-09-19 13:55:15]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-09-19 13:56:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/587683002/1

[commit-bot: I haz the power, 2014-09-19 15:04:52]

Committed patchset #1 (id:1) as 182332