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 dffb6999ea46b40e65a624eb5517db9e241a1a2c..0780d0a99b2b3050a3eb3b945fc25b0547a91fc9 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -144,6 +144,7 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
     , m_hasSelfPaintingLayerDescendantDirty(false)
     , m_hasOutOfFlowPositionedDescendant(false)
     , m_hasOutOfFlowPositionedDescendantDirty(true)
+    , m_forceNeedsCompositedScrolling(DoNotForceCompositedScrolling)
     , m_needsCompositedScrolling(false)
     , m_descendantsAreContiguousInStackingOrder(false)
     , m_descendantsAreContiguousInStackingOrderDirty(true)
@@ -2051,6 +2052,15 @@ bool RenderLayer::usesCompositedScrolling() const
 
 bool RenderLayer::needsCompositedScrolling() const
 {
+    switch (m_forceNeedsCompositedScrolling) {
+    case DoNotForceCompositedScrolling:
+        return m_needsCompositedScrolling;
+    case ForceCompositedScrollingOn:
+        return true;
+    case ForceCompositedScrollingOff:
+        return false;
+    }
+

[Julien - ping for review, 2013/05/06 22:56:59]

ASSERT_NOT_REACHED()

[Ian Vollick, 2013/05/08 13:10:58]

Done.

     return m_needsCompositedScrolling;
 }
 
@@ -2084,6 +2094,21 @@ void RenderLayer::updateNeedsCompositedScrolling()
 
     m_needsCompositedScrolling = needsCompositedScrolling;
 
+    // Note, the z-order lists may need to be rebuilt, but our code guarantees
+    // that we have not affected stacking, so we will not dirty
+    // m_canBePromotedToStackingContainer for either us or our stacking context
+    // or container.
+    didUpdateNeedsCompositedScrolling();
+}
+
+void RenderLayer::setForceNeedsCompositedScrolling(RenderLayer::ForceNeedsCompositedScrollingMode mode)
+{
+    m_forceNeedsCompositedScrolling = mode;
+    didUpdateNeedsCompositedScrolling();
+}
+
+void RenderLayer::didUpdateNeedsCompositedScrolling()
+{
     updateIsNormalFlowOnly();
     updateSelfPaintingLayer();