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

third_party/WebKit/Source/web/WebViewImpl.h

Index: third_party/WebKit/Source/web/WebViewImpl.h
diff --git a/third_party/WebKit/Source/web/WebViewImpl.h b/third_party/WebKit/Source/web/WebViewImpl.h
index cbf00bed5b40402b07f6d1a4c8455f616e04fe59..eab0ba9f8aaea30d19de9bce7065880ff5c62b20 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.h
+++ b/third_party/WebKit/Source/web/WebViewImpl.h
@@ -210,6 +210,8 @@ public:
 
     float deviceScaleFactor() const override;
     void setDeviceScaleFactor(float) override;
+    void setZoomFactorForDeviceScaleFactor(float) override;
+
     void setDeviceColorProfile(const WebVector<char>&) override;
     void resetDeviceColorProfile() override;
 
@@ -652,6 +654,9 @@ private:
 
     double m_maximumZoomLevel;
 
+    // Additional zoom factor used to scale the content by device scale factor.
+    double m_zoomFactorForDeviceScaleFactor;
+
     // This value, when multiplied by the font scale factor, gives the maximum
     // page scale that can result from automatic zooms.
     float m_maximumLegibleScale;