RenderLayer::clipParent() should avoid calling clippedByAncestor unless needed

Calling clippedByAncestor is not free, we should avoid calling it unless
the compositingReasons() say we need to.

R=ojan@chromium.org,vollick@chromium.org,abarth@chromium.org

Review URL: https://codereview.chromium.org/183263003

git-svn-id: svn://svn.chromium.org/blink/trunk@168234 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/rendering/RenderLayer.cpp

@@ -1680,15 +1680,11 @@ RenderLayer* RenderLayer::scrollParent() const
 
 RenderLayer* RenderLayer::clipParent() const
 {
-    const bool needsAncestorClip = compositor()->clippedByAncestor(this);
-
-    RenderLayer* clipParent = 0;
-    if ((compositingReasons() & CompositingReasonOutOfFlowClipping) && !needsAncestorClip) {
+    if (compositingReasons() & CompositingReasonOutOfFlowClipping && !compositor()->clippedByAncestor(this)) {
         if (RenderObject* containingBlock = renderer()->containingBlock())
-            clipParent = containingBlock->enclosingLayer()->enclosingCompositingLayer();
+            return containingBlock->enclosingLayer()->enclosingCompositingLayer();
     }
-
-    return clipParent;
+    return 0;
 }
 
 void RenderLayer::didUpdateNeedsCompositedScrolling()