Make scrollbars in pinch viewport invisible until compositor makes them visible.

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 290 matching lines @@
     scrollbarMargin = ScrollbarTheme::theme()->scrollbarMargin();
 #endif
 
     if (!webScrollbarLayer) {
         ScrollingCoordinator* coordinator = frameHost().page().scrollingCoordinator();
         ASSERT(coordinator);
         ScrollbarOrientation webcoreOrientation = isHorizontal ? HorizontalScrollbar : VerticalScrollbar;
         webScrollbarLayer = coordinator->createSolidColorScrollbarLayer(webcoreOrientation, thumbThickness, scrollbarMargin, false);
 
         webScrollbarLayer->setClipLayer(m_innerViewportContainerLayer->platformLayer());
+
+        // The compositor will control the scrollbar's visibility. Set to invisible by defualt
+        // so scrollbars don't show up in layout tests.
+        webScrollbarLayer->layer()->setOpacity(0);

[gmorrita, 2014/10/30 18:48:47]

Apparently having some API on WebScrollbarLayer looks better than touching
layer() directly?

+
         scrollbarGraphicsLayer->setContentsToPlatformLayer(webScrollbarLayer->layer());
         scrollbarGraphicsLayer->setDrawsContent(false);
     }
 
     int xPosition = isHorizontal ? 0 : m_innerViewportContainerLayer->size().width() - scrollbarThickness;
     int yPosition = isHorizontal ? m_innerViewportContainerLayer->size().height() - scrollbarThickness : 0;
     int width = isHorizontal ? m_innerViewportContainerLayer->size().width() - scrollbarThickness : scrollbarThickness;
     int height = isHorizontal ? scrollbarThickness : m_innerViewportContainerLayer->size().height() - scrollbarThickness;
 
     // Use the GraphicsLayer to position the scrollbars.
@@ skipping 187 matching lines @@
     } else if (graphicsLayer == m_overlayScrollbarVertical.get()) {
         name =  "Overlay Scrollbar Vertical Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink