[DevTools] Use device metrics emulation implemented in content.

Source/web/WebViewImpl.h

Index: Source/web/WebViewImpl.h
diff --git a/Source/web/WebViewImpl.h b/Source/web/WebViewImpl.h
index 04be195d68cc14dd5655e2c932d36148bebea51d..1feaf7f8534454e53ab5841b4c736546a5d444f7 100644
--- a/Source/web/WebViewImpl.h
+++ b/Source/web/WebViewImpl.h
@@ -630,6 +630,8 @@ private:
     void doPixelReadbackToCanvas(WebCanvas*, const WebCore::IntRect&);
     void reallocateRenderer();
     void updateLayerTreeViewport();
+    void updateRootLayerTransform();
+    void updateLayerTreeDeviceScaleFactor();
 
     // Helper function: Widens the width of |source| by the specified margins
     // while keeping it smaller than page width.
@@ -729,6 +731,9 @@ private:
 
     bool m_ignoreInputEvents;
 
+    float m_compositorDeviceScaleFactorOverride;
+    float m_rootLayerScale;
+
     // Webkit expects keyPress events to be suppressed if the associated keyDown
     // event was handled. Safari implements this behavior by peeking out the
     // associated WM_CHAR event if the keydown was handled. We emulate