Add touchpad pinch zoom support to Blink

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 206 matching lines @@
     }
 
     if (!valuesChanged)
         return;
 
     mainFrame()->loader().saveScrollState();
 
     clampToBoundaries();
 }
 
+bool PinchViewport::magnifyScaleAroundAnchor(float magnifyDelta, const FloatPoint& anchor)
+{
+    const float oldPageScale = scale();
+    const float newPageScale = frameHost().chrome().client().clampPageScaleFactorToLimits(
+        magnifyDelta * oldPageScale);
+    if (newPageScale == oldPageScale)
+        return false;
+
+    // Keep the center-of-pinch anchor in a stable position over the course
+    // of the magnify.
+    FloatPoint anchorAtOldScale = anchor.scaledBy(1.f / oldPageScale);
+    FloatPoint anchorAtNewScale = anchor.scaledBy(1.f / newPageScale);
+    FloatSize anchorDelta = anchorAtOldScale - anchorAtNewScale;
+
+    // First try to use the anchor's delta to scroll the FrameView.
+    FloatSize anchorDeltaUnusedByScroll = anchorDelta;
+    FrameView* view = mainFrame()->view();

[aelias_OOO_until_Jul13, 2015/02/03 19:10:11]

I'm not confident FrameView is guaranteed to exist when a pinch event arrives. 
Please add an early-return at the top of this method:

if (!mainFrame() || !mainFrame()->view())
  return false;

+    DoublePoint oldPosition = view->scrollPositionDouble();
+    view->scrollBy(DoubleSize(anchorDelta.width(), anchorDelta.height()));
+    DoublePoint newPosition = view->scrollPositionDouble();
+    anchorDeltaUnusedByScroll = anchorDelta - toFloatSize(newPosition - oldPosition);
+
+    // Manually bubble any remaining anchor delta up to the pinch viewport.
+    FloatPoint newLocation(location() + anchorDeltaUnusedByScroll);
+    setScaleAndLocation(newPageScale, newLocation);
+    return true;
+}
+
 // Modifies the top of the graphics layer tree to add layers needed to support
 // the inner/outer viewport fixed-position model for pinch zoom. When finished,
 // the tree will look like this (with * denoting added layers):
 //
 // *rootTransformLayer
 //  +- *innerViewportContainerLayer (fixed pos container)
 //  |   +- *overscrollElasticityLayer
 //  |       +- *pageScaleLayer
 //  |           +- *innerViewportScrollLayer
 //  |               +-- overflowControlsHostLayer (root layer)
@@ skipping 310 matching lines @@
     } else if (graphicsLayer == m_rootTransformLayer) {
         name =  "Root Transform Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink