[Pinch-to-zoom] Moved scale factor into PinchViewport

Source/core/frame/FrameHost.h

Index: Source/core/frame/FrameHost.h
diff --git a/Source/core/frame/FrameHost.h b/Source/core/frame/FrameHost.h
index 5f2aaaf441396273c80070370dec692b3f2fa48b..f570105b2938a815812fefb8389dc2f2c5d62446 100644
--- a/Source/core/frame/FrameHost.h
+++ b/Source/core/frame/FrameHost.h
@@ -72,13 +72,13 @@ public:
     // This value does not account for Page zoom, use LocalFrame::devicePixelRatio instead.
     float deviceScaleFactor() const;
 
-    PinchViewport& pinchViewport();
+    PinchViewport& pinchViewport() const;
 
 private:
     explicit FrameHost(Page&);
 
     Page& m_page;
-    PinchViewport m_pinchViewport;
+    const OwnPtr<PinchViewport> m_pinchViewport;
 };
 
 }