Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/scrollbar_layer_impl.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_impl.h"
 
 #include "cc/layer_tree_impl.h"
 #include "cc/layer_tree_settings.h"
 #include "cc/quad_sink.h"
 #include "cc/scrollbar_animation_controller.h"
@@ skipping 120 matching lines @@
     scrollbarLayer->setScrollbarData(&m_scrollbar);
     scrollbarLayer->setThumbSize(m_thumbSize);
 
     scrollbarLayer->setBackTrackResourceId(m_backTrackResourceId);
     scrollbarLayer->setForeTrackResourceId(m_foreTrackResourceId);
     scrollbarLayer->setThumbResourceId(m_thumbResourceId);
 }
 
 void ScrollbarLayerImpl::appendQuads(QuadSink& quadSink, AppendQuadsData& appendQuadsData)
 {
+    // TODO(wjmaclean) Make quads respect layer opacity for fading scrollbars in/out
+    // via opacity animations.
+    if (!opacity())
+        return;
+
     bool premultipledAlpha = true;
     bool flipped = false;
     gfx::PointF uvTopLeft(0.f, 0.f);
     gfx::PointF uvBottomRight(1.f, 1.f);
     gfx::Rect boundsRect(gfx::Point(), bounds());
     gfx::Rect contentBoundsRect(gfx::Point(), contentBounds());
 
     SharedQuadState* sharedQuadState = quadSink.useSharedQuadState(createSharedQuadState());
     appendDebugBorderQuad(quadSink, sharedQuadState, appendQuadsData);
 
@@ skipping 138 matching lines @@
 {
     return m_owner->m_isCustomScrollbar;
 }
 
 const char* ScrollbarLayerImpl::layerTypeAsString() const
 {
     return "ScrollbarLayer";
 }
 
 }  // namespace cc