Clean up the way layout tests force elements to opt in/out of composited scrolling.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index e8186688e1b0e2a7a5c1c73199211117e76dfad0..cc2967fc3438809203aae2457ed9c3ccbd279a2d 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -484,8 +484,8 @@ void RenderLayer::dirtyAncestorChainHasSelfPaintingLayerDescendantStatus()
 bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
 {
     return renderer()->frame()
-        && renderer()->frame()->page()
-        && renderer()->frame()->page()->settings()->acceleratedCompositingForOverflowScrollEnabled();
+        && renderer()->frame()->page()->settings()

[Julien - ping for review, 2013/05/13 20:48:56]

I don't think this is right: the page can be NULL, not the Setings themselves
(their are an OwnPtr populated inside Page constructor).

Why you don't just use Document::settings as it would avoid the need for such
custom code?

Settings* settings = renderer()->document()->settings();
return settings && (settings->acceleratedCompositingForOverflowScrollEnabled()
!= AcceleratedCompositingForOverflowScrollDisabled);

+        && (renderer()->frame()->page()->settings()->acceleratedCompositingForOverflowScrollEnabled() != AcceleratedCompositingForOverflowScrollDisabled);
 }
 
 // If we are a stacking container, then this function will determine if our
@@ -567,7 +567,7 @@ bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
 //  And we would conclude that C could be promoted.
 void RenderLayer::updateDescendantsAreContiguousInStackingOrder()
 {
-    if (!m_descendantsAreContiguousInStackingOrderDirty || !isStackingContext() || !acceleratedCompositingForOverflowScrollEnabled())
+    if (!m_descendantsAreContiguousInStackingOrderDirty || !isStackingContext() || acceleratedCompositingForOverflowScrollEnabled() == AcceleratedCompositingForOverflowScrollDisabled)
         return;
 
     ASSERT(!m_normalFlowListDirty);
@@ -2057,6 +2057,20 @@ bool RenderLayer::usesCompositedScrolling() const
 
 bool RenderLayer::needsCompositedScrolling() const
 {
+    AcceleratedCompositingForOverflowScrollEnabledMode mode = AcceleratedCompositingForOverflowScrollDisabled;
+    if (renderer()->frame() && renderer()->frame()->page()->settings())

[Julien - ping for review, 2013/05/13 20:48:56]

Same comment here about NULL-checking settings.

+        mode = renderer()->frame()->page()->settings()->acceleratedCompositingForOverflowScrollEnabled();
+
+    switch (mode) {
+    case AcceleratedCompositingForOverflowScrollEnabled:
+        return m_needsCompositedScrolling;
+    case AcceleratedCompositingForOverflowScrollDisabled:
+        return false;
+    case AcceleratedCompositingForOverflowScrollAlwaysOn:
+        return true;
+    }
+
+    ASSERT_NOT_REACHED();
     return m_needsCompositedScrolling;
 }