Only do full tree sync if tree is actually changed, otherwise just push properties

cc/scrollbar_layer.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/scrollbar_layer.h"
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "cc/layer_painter.h"
 #include "cc/layer_tree_host.h"
@@ skipping 22 matching lines @@
     , m_geometry(geometry.Pass())
     , m_scrollLayerId(scrollLayerId)
     , m_textureFormat(GL_INVALID_ENUM)
 {
 }
 
 ScrollbarLayer::~ScrollbarLayer()
 {
 }
 
+void ScrollbarLayer::setScrollLayerId(int id)
+{
+    if (id == m_scrollLayerId)
+        return;
+
+    m_scrollLayerId = id;
+    setNeedsFullTreeSync();
+}
+
 int ScrollbarLayer::maxTextureSize() {
     DCHECK(layerTreeHost());
     return layerTreeHost()->rendererCapabilities().maxTextureSize;
 }
 
 float ScrollbarLayer::clampScaleToMaxTextureSize(float scale) {
     // If the scaled contentBounds() is bigger than the max texture size of the
     // device, we need to clamp it by rescaling, since contentBounds() is used
     // below to set the texture size.
     gfx::Size scaledBounds = computeContentBoundsForScale(scale, scale);
@@ skipping 242 matching lines @@
         updatePart(m_foreTrackUpdater.get(), m_foreTrack.get(), contentRect, queue, stats);
 
     // Consider the thumb to be at the origin when painting.
     WebKit::WebRect thumbRect = m_geometry->thumbRect(m_scrollbar.get());
     gfx::Rect originThumbRect = scrollbarLayerRectToContentRect(gfx::Rect(0, 0, thumbRect.width, thumbRect.height));
     if (!originThumbRect.IsEmpty())
         updatePart(m_thumbUpdater.get(), m_thumb.get(), originThumbRect, queue, stats);
 }
 
 }  // namespace cc