Made root translation layer the root when using pinch virtual viewport.

Source/core/frame/PinchViewport.cpp

Index: Source/core/frame/PinchViewport.cpp
diff --git a/Source/core/frame/PinchViewport.cpp b/Source/core/frame/PinchViewport.cpp
index b5897b3502b973b5d61a0cf6c850b303912cc289..c11f62758b939c50674fe58fb3d17ca72bfc6a68 100644
--- a/Source/core/frame/PinchViewport.cpp
+++ b/Source/core/frame/PinchViewport.cpp
@@ -195,11 +195,11 @@ void PinchViewport::setScale(float scale)
 // the inner/outer viewport fixed-position model for pinch zoom. When finished,
 // the tree will look like this (with * denoting added layers):
 //
-// *innerViewportContainerLayer (fixed pos container)
-//  +- *pageScaleLayer
-//  |   +- *innerViewportScrollLayer
-//  |       +-- overflowControlsHostLayer (root layer)
-//  |           +-- rootTransformLayer (optional)
+// *rootTransformLayer
+//  +- *innerViewportContainerLayer (fixed pos container)
+//      +- *pageScaleLayer
+//  |       +- *innerViewportScrollLayer
+//  |           +-- overflowControlsHostLayer (root layer)
 //  |               +-- outerViewportContainerLayer (fixed pos container) [frame container layer in RenderLayerCompositor]
 //  |               |   +-- outerViewportScrollLayer [frame scroll layer in RenderLayerCompositor]
 //  |               |       +-- content layers ...
@@ -226,6 +226,8 @@ void PinchViewport::attachToLayerTree(GraphicsLayer* currentLayerTreeRoot, Graph
             && !m_pageScaleLayer
             && !m_innerViewportContainerLayer);
 
+        // FIXME: The root transform layer should only be created on demand.
+        m_rootTransformLayer = GraphicsLayer::create(graphicsLayerFactory, this);
         m_innerViewportContainerLayer = GraphicsLayer::create(graphicsLayerFactory, this);
         m_pageScaleLayer = GraphicsLayer::create(graphicsLayerFactory, this);
         m_innerViewportScrollLayer = GraphicsLayer::create(graphicsLayerFactory, this);
@@ -245,6 +247,7 @@ void PinchViewport::attachToLayerTree(GraphicsLayer* currentLayerTreeRoot, Graph
             m_innerViewportContainerLayer->platformLayer());
         m_innerViewportScrollLayer->platformLayer()->setUserScrollable(true, true);
 
+        m_rootTransformLayer->addChild(m_innerViewportContainerLayer.get());
         m_innerViewportContainerLayer->addChild(m_pageScaleLayer.get());
         m_pageScaleLayer->addChild(m_innerViewportScrollLayer.get());
         m_innerViewportContainerLayer->addChild(m_overlayScrollbarHorizontal.get());