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

cc/layer_impl.cc

 // Copyright 2011 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/layer_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "base/stringprintf.h"
 #include "base/values.h"
 #include "cc/animation_registrar.h"
@@ skipping 35 matching lines @@
     , m_drawCheckerboardForMissingTiles(false)
     , m_drawsContent(false)
     , m_forceRenderSurface(false)
     , m_isContainerForFixedPositionLayers(false)
     , m_fixedToContainerLayer(false)
     , m_drawDepth(0)
 #ifndef NDEBUG
     , m_betweenWillDrawAndDidDraw(false)
 #endif
 {
-    DCHECK(m_layerId > 0);
+    DCHECK(m_layerId > 0 || m_layerId == -1 || m_layerId == -2);

[Ian Vollick, 2013/01/15 20:33:48]

Same here and anywhere else this may crop up.

     DCHECK(m_layerTreeImpl);
     m_layerTreeImpl->RegisterLayer(this);
     AnimationRegistrar* registrar = m_layerTreeImpl->animationRegistrar();
     m_layerAnimationController = registrar->GetAnimationControllerForId(m_layerId);
     m_layerAnimationController->addObserver(this);
 }
 
 LayerImpl::~LayerImpl()
 {
 #ifndef NDEBUG
@@ skipping 790 matching lines @@
         if (m_layerTreeImpl->settings().useLinearFadeScrollbarAnimator)
             m_scrollbarAnimationController = createScrollbarAnimationControllerWithFade(this);
         else
             m_scrollbarAnimationController = ScrollbarAnimationController::create(this);
     }
     m_scrollbarAnimationController->setVerticalScrollbarLayer(scrollbarLayer);
     m_scrollbarAnimationController->updateScrollOffset(this);
 }
 
 }  // namespace cc