Move computation of RenderLayer::isUnclippedDescendant into CompositingPropertyUpdater

Source/core/rendering/compositing/CompositingReasonFinder.cpp

Index: Source/core/rendering/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/rendering/compositing/CompositingReasonFinder.cpp b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
index 8f2bda03a963d6e6c4649a077ae32130dd4ce6f5..e78fb944962afae68ba34ae0198529c3113a3103 100644
--- a/Source/core/rendering/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
@@ -142,7 +142,10 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
     RenderObject* renderer = layer->renderer();
 
     if (hasOverflowScrollTrigger()) {
-        if (layer->isUnclippedDescendant())
+        // IsUnclippedDescendant is only actually stale during the chicken/egg code path.
+        // FIXME: Use ancestorDependentProperties().isUnclippedDescendant to ASSERT that
+        // this value isn't stale.

[Ian Vollick, 2014/06/01 19:25:36]

What would you think of using the disabler here?

[abarth-chromium, 2014/06/01 19:49:36]

There isn't a disabler for this ASSERT because it's guarding the inputs to the
computing update rather than the outputs.  Hopefully there won't be any more
violations...

This would be clearer if we renamed ancestorDependentProperties() to
compositingInputs()    :)

+        if (layer->potentiallyStaleIsUnclippedDescendant())
             directReasons |= CompositingReasonOutOfFlowClipping;
 
         if (layer->scrollParent())