Made double-tap zoom work in pinch virtual viewport mode. (Blink-side)

Source/core/frame/PinchViewport.h

Index: Source/core/frame/PinchViewport.h
diff --git a/Source/core/frame/PinchViewport.h b/Source/core/frame/PinchViewport.h
index b934131c2f5a7c872596a5efa21e9a4168851200..f12b774e0ea49a603e99d93a90d920c0c52443b9 100644
--- a/Source/core/frame/PinchViewport.h
+++ b/Source/core/frame/PinchViewport.h
@@ -93,6 +93,10 @@ public:
     // rotation on Android or window resize elsewhere).
     void mainFrameDidChangeSize();
 
+    // Sets scale without ensuring the viewport is within its bounds. Useful for
+    // situations when scale and offset are changing at the same time, e.g. during
+    // a compositor commit.
+    void setScaleWithoutClampingViewportOffset(float);
     void setScale(float);
     float scale() const { return m_scale; }
 
@@ -110,6 +114,11 @@ public:
     // top-level document is centered in the viewport. This method will avoid
     // scrolling the pinch viewport unless necessary.
     void scrollIntoView(const FloatRect&);
+
+    // Clamp the given point, in document coordinates, to the maximum/minimum
+    // scroll extents of the viewport within the document.
+    IntPoint clampDocumentOffsetAtScale(const IntPoint&, float);
+
 private:
     // ScrollableArea implementation
     virtual bool isActive() const OVERRIDE { return false; }