Don't always make backing store for -webkit-backface-visibility:hidden

Don't always make backing store for -webkit-backface-visibility:hidden
https://bugs.webkit.org/show_bug.cgi?id=132420

Reviewed by Sam Weinig.

Source/WebCore:
Previously, -webkit-backface-visibility:hidden unconditionally created
compositing layers with backing store. This results in high memory use
on pages with this style applied to many elements (a cargo-cult "optimization").

Fix by only having -webkit-backface-visibility:hidden create compositing layers
if some ancestor has a 3D transform. That's the only scenario in which the
element can be flipped around to reveal the back side, so the only time we need
to do compositing for this property. In future, we could be smarter, and only
consider 3D transforms in the current preserve-3d context.

Tests: compositing/backing/backface-visibility-in-3dtransformed.html
       compositing/backing/backface-visibility-in-transformed.html
       compositing/backing/backface-visibility.html

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::RenderLayer):
(WebCore::RenderLayer::updateLayerPositions):
(WebCore::RenderLayer::hitTestLayer):
* rendering/RenderLayer.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::requiresCompositingLayer):
(WebCore::RenderLayerCompositor::requiresOwnBackingStore):
(WebCore::RenderLayerCompositor::requiresCompositingForBackfaceVisibility):
* rendering/RenderLayerCompositor.h:

LayoutTests:
Dump layers for elements with backface-visibility: hidden with various types
of ancestors.

* compositing/backing/backface-visibility-expected.txt: Added.
* compositing/backing/backface-visibility-in-3dtransformed-expected.txt: Added.
* compositing/backing/backface-visibility-in-3dtransformed.html: Added.
* compositing/backing/backface-visibility-in-transformed-expected.txt: Added.
* compositing/backing/backface-visibility-in-transformed.html: Added.
* compositing/backing/backface-visibility.html: Added.
* inspector-protocol/layers/layers-anonymous.html: Don't use backface-visibility
for force a layer.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168119 268f45cc-cd09-0410-ab3c-d52691b4dbfc