Add Blink-side flag for enabling/disabling layer squashing.

Source/core/rendering/RenderLayerCompositor.cpp

Index: Source/core/rendering/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/RenderLayerCompositor.cpp b/Source/core/rendering/RenderLayerCompositor.cpp
index a6cd806288b66e3ede0e92a9c7490989cfa85c7d..2708ddfa0a50dccbf53ac9de037aba55a2bceb53 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -273,6 +273,14 @@ void RenderLayerCompositor::cacheAcceleratedCompositingFlags()
     m_forceCompositingMode = forceCompositingMode;
 }
 
+bool RenderLayerCompositor::isLayerSquashingEnabled() const
+{
+    if (Settings* settings = m_renderView->document().settings())
+        return settings->isLayerSquashingEnabled();
+
+    return false;
+}
+
 bool RenderLayerCompositor::canRender3DTransforms() const
 {
     return hasAcceleratedCompositing() && (m_compositingTriggers & ChromeClient::ThreeDTransformTrigger);