HTMLMediaElement will not unthrottle page when playback stops for nreasons other than user-initiated pause.
https://bugs.webkit.org/show_bug.cgi?id=117016

Reviewed by Oliver Hunt.

Add a new class PageActivityAssertionToken to allow HTMLMediaElement to decouple
knowledge of and control over the lifetime of PageThrottler. The new class will
have weak references to and from the PageThrottler so that holders of the token
will not need to care if the Page or PageThrottler has been destroyed.

HTMLMediaElement will create one of these PageActivityAssertionTokens when playback
begins and destroy it when playback stops for any reason, or when the element is
destroyed.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::~HTMLMediaElement):
(WebCore::HTMLMediaElement::playInternal):
(WebCore::HTMLMediaElement::pauseInternal):
(WebCore::HTMLMediaElement::playbackProgressTimerFired):
(WebCore::HTMLMediaElement::updatePlayState):
* html/HTMLMediaElement.h:
* page/Page.cpp:
(WebCore::createActivityToken): Added simple factory method.
* page/Page.h:
* page/PageActivityAssertionToken.cpp:
(WebCore::PageActivityAssertionToken::PageActivityAssertionToken): Call addActivityToken();
(WebCore::PageActivityAssertionToken::~PageActivityAssertionToken): Call removeActivityToken();
(WebCore::PageActivityAssertionToken::invalidate): Clear m_throttler.
* page/PageActivityAssertionToken.h:
* page/PageThrottler.cpp:
(WebCore::PageThrottler::~PageThrottler): Invalidate all outstanding tokens.
(WebCore::PageThrottler::addActivityToken): Bump the activity count.
(WebCore::PageThrottler::removeActivityToken): Lower the activity count.
* page/PageThrottler.h:

Add the new files to the various build systems:
* CMakeLists.txt:
* GNUmakefile.list.am:
* Target.pri:
* WebCore.vcproj/WebCore.vcproj:
* WebCore.vcxproj/WebCore.vcxproj:
* WebCore.vcxproj/WebCore.vcxproj.filters:
* WebCore.xcodeproj/project.pbxproj:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150971 268f45cc-cd09-0410-ab3c-d52691b4dbfc