<http://webkit.org/b/68421> Stop calling UpdateSystemActivity in places where we hold power assertions that achieve the same effect

On SnowLeopard and newer it's not necessary to call UpdateSystemActivity to prevent the screensaver from kicking
in as the NoDisplaySleep assertion now has that effect. It's also not necessary to hold both a NoDisplaySleep and
a NoIdleSleep assertion as the latter is implied by the former.

Source/WebCore:

Since there were multiple copies of the same code to disable display sleep spread across WebCore, WebKit and WebKit2
this patch takes the approach of moving this functionality to a new DisplaySleepDisabler class that lives in WebCore.
A class is used as it makes it easier to reason about the lifetime of the assertions and to ensure that the assertions will
always be released when their owning objects go away.

Reviewed by Dan Bernstein.

* WebCore.exp.in:
* WebCore.xcodeproj/project.pbxproj:
* platform/mac/DisplaySleepDisabler.cpp:
(WebCore::DisplaySleepDisabler::DisplaySleepDisabler): Register for the NoDisplaySleep assertion, and start our
system activity timer when on Leopard.
(WebCore::DisplaySleepDisabler::~DisplaySleepDisabler): Unregister for the assertion.
(WebCore::DisplaySleepDisabler::systemActivityTimerFired): Call UpdateSystemActivity to prevent the screensaver from
triggering on Leopard.
* platform/mac/DisplaySleepDisabler.h:
(WebCore::DisplaySleepDisabler::create):
* platform/mac/WebVideoFullscreenController.h:
* platform/mac/WebVideoFullscreenController.mm:
(-[WebVideoFullscreenController dealloc]):
(-[WebVideoFullscreenController setupVideoOverlay:]): Remove the now-unnecessary explicit use of the WebCore namespace.
(-[WebVideoFullscreenController mediaElement]): Ditto.
(-[WebVideoFullscreenController setMediaElement:]): Ditto.
(-[WebVideoFullscreenController windowDidExitFullscreen]): Ditto.
(-[WebVideoFullscreenController updatePowerAssertions]): Create a DisplaySleepDisabler instance when transitioning from permitting
display sleep to disabling display sleep. Clear out our reference when sleep should be permitted, destroying any
disabler that we own.

Source/WebKit/mac:

Adopt the DisplaySleepDisabler class that was added to WebCore and remove all of duplicated logic from WebFullScreenController.

Reviewed by Dan Bernstein.

* WebView/WebFullScreenController.h:
* WebView/WebFullScreenController.mm:
(-[WebFullScreenController dealloc]):
(-[WebFullScreenController _updatePowerAssertions]): Create a DisplaySleepDisabler instance when transitioning from permitting
display sleep to disabling display sleep. Clear out our reference when sleep should be permitted, destroying any
disabler that we own.

Source/WebKit2:

Adopt the DisplaySleepDisabler class that was added to WebCore and remove all of duplicated logic from WKFullScreenWindowController.

Reviewed by Dan Bernstein.

* UIProcess/mac/WKFullScreenWindowController.h:
* UIProcess/mac/WKFullScreenWindowController.mm:
(-[WKFullScreenWindowController _updatePowerAssertions]): Create a DisplaySleepDisabler instance when transitioning from permitting
display sleep to disabling display sleep. Clear out our reference when sleep should be permitted, destroying any
disabler that we own. Also adds a FIXME about an error that was noticed while working in this code.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95513 268f45cc-cd09-0410-ab3c-d52691b4dbfc