Don't throttle layer flushes when the main resource is a GIF
https://bugs.webkit.org/show_bug.cgi?id=134650

Source/WebCore: 
<rdar://problem/17490712>

Reviewed by Simon Fraser.
        
Avoid throttling big image animations.

* page/FrameView.cpp:
(WebCore::determineLayerFlushThrottleState):
        
    Disable for image documents.

(WebCore::FrameView::disableLayerFlushThrottlingTemporarilyForInteraction):
(WebCore::FrameView::updateLayerFlushThrottling):
        
    Refactor a bit.

* page/LayerFlushThrottleState.h:
        
    Rename the flag.

Source/WebKit2: 

Reviewed by Simon Fraser.

* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::adjustLayerFlushThrottling):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@170830 268f45cc-cd09-0410-ab3c-d52691b4dbfc