Introduce painted-device-scale-factor and use it when --enable-use-zoom-for-dsf is specified.

third_party/WebKit/public/web/WebView.h

Index: third_party/WebKit/public/web/WebView.h
diff --git a/third_party/WebKit/public/web/WebView.h b/third_party/WebKit/public/web/WebView.h
index ccd46ce923ab392a5ec3aa6c8e7f44ebcbcfe5de..f076de7ccb7905fcc4e048157e40f3505d0c8949 100644
--- a/third_party/WebKit/public/web/WebView.h
+++ b/third_party/WebKit/public/web/WebView.h
@@ -293,6 +293,9 @@ public:
     // Sets the ratio as computed by computePageScaleConstraints.
     virtual void setDeviceScaleFactor(float) = 0;
 
+    // Enables the mode to use zoom to implement device scale factor.
+    virtual void enableUseZoomForDeviceScaleFactor() = 0;
+
     // Set and reset the device color profile.
     virtual void setDeviceColorProfile(const WebVector<char>&) = 0;
     virtual void resetDeviceColorProfile() = 0;