Dynamically triggered subframe loads are causing tile churn

Dynamically triggered subframe loads are causing tile churn
https://bugs.webkit.org/show_bug.cgi?id=114440

Reviewed by Andreas Kling.

In some cases we switch in and out of minimal tiles multiple times during page loading. This
creates tile churn where we create speculative tiles and then throw them out. This can be
caused by various reasons including dynamically created subframes.

* page/FrameView.cpp:
(WebCore::FrameView::setIsVisuallyNonEmpty):

    Stay in minimal tiles modes until there is some visual contents. This avoids switching out from minimal
    tiles while awaiting response for the initial requests.

* page/FrameView.h:
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::RenderLayerBacking):
(WebCore::computeTileCoverage):
* rendering/RenderLayerBacking.h:
(WebCore::RenderLayerBacking::didSwitchToFullTileCoverageDuringLoading):
(WebCore::RenderLayerBacking::setDidSwitchToFullTileCoverageDuringLoading):

    Only switch into minimal coverage mode once per page load.

(RenderLayerBacking):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@148432 268f45cc-cd09-0410-ab3c-d52691b4dbfc