Add a trace for RenderLayer::updateLayerPositionsAfterLayout.

I'm seeing this taking up a big portion of FrameView::layout in my trace.

Review URL: https://codereview.chromium.org/138823003

git-svn-id: svn://svn.chromium.org/blink/trunk@165106 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -236,6 +236,7 @@ LayoutPoint RenderLayer::computeOffsetFromRoot(bool& hasLayerOffset) const
 
 void RenderLayer::updateLayerPositionsAfterLayout(const RenderLayer* rootLayer, UpdateLayerPositionsFlags flags)
 {
+    TRACE_EVENT0("blink_rendering", "RenderLayer::updateLayerPositionsAfterLayout");
     RenderGeometryMap geometryMap(UseTransforms);
     if (this != rootLayer)
         geometryMap.pushMappingsToAncestor(parent(), 0);