Set track_start on solid color layers to the margin.

Source/core/frame/PinchViewport.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 184 matching lines @@
         m_overlayScrollbarHorizontal.get() : m_overlayScrollbarVertical.get();
     OwnPtr<WebScrollbarLayer>& webScrollbarLayer = isHorizontal ?
         m_webOverlayScrollbarHorizontal : m_webOverlayScrollbarVertical;
 
     const int overlayScrollbarThickness = m_frameHost.settings().pinchOverlayScrollbarThickness();
 
     if (!webScrollbarLayer) {
         ScrollingCoordinator* coordinator = m_frameHost.page().scrollingCoordinator();
         ASSERT(coordinator);
         ScrollbarOrientation webcoreOrientation = isHorizontal ? HorizontalScrollbar : VerticalScrollbar;
-        webScrollbarLayer = coordinator->createSolidColorScrollbarLayer(webcoreOrientation, overlayScrollbarThickness, false);
+        webScrollbarLayer = coordinator->createSolidColorScrollbarLayer(webcoreOrientation, overlayScrollbarThickness, 0, false);
 
         webScrollbarLayer->setClipLayer(m_innerViewportContainerLayer->platformLayer());
         scrollbarGraphicsLayer->setContentsToPlatformLayer(webScrollbarLayer->layer());
         scrollbarGraphicsLayer->setDrawsContent(false);
     }
 
     int xPosition = isHorizontal ? 0 : m_innerViewportContainerLayer->size().width() - overlayScrollbarThickness;
     int yPosition = isHorizontal ? m_innerViewportContainerLayer->size().height() - overlayScrollbarThickness : 0;
     int width = isHorizontal ? m_innerViewportContainerLayer->size().width() - overlayScrollbarThickness : overlayScrollbarThickness;
     int height = isHorizontal ? overlayScrollbarThickness : m_innerViewportContainerLayer->size().height() - overlayScrollbarThickness;
@@ skipping 128 matching lines @@
     } else if (graphicsLayer == m_overlayScrollbarVertical.get()) {
         name =  "Overlay Scrollbar Vertical Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore