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

content/renderer/render_view_impl.h

Index: content/renderer/render_view_impl.h
diff --git a/content/renderer/render_view_impl.h b/content/renderer/render_view_impl.h
index 591cd6b4d0c681165bf1e8338d8f18d02a84c128..73372befb3cf12a3a4d962a34f24bb24d3a9f081 100644
--- a/content/renderer/render_view_impl.h
+++ b/content/renderer/render_view_impl.h
@@ -762,6 +762,8 @@ class CONTENT_EXPORT RenderViewImpl
   void UpdateThemePrefs() {}
 #endif
 
+  void UpdateDeviceScaleFactor();
+
   // ---------------------------------------------------------------------------
   // ADDING NEW FUNCTIONS? Please keep private functions alphabetized and put
   // it in the same order in the .cc file as it was in the header.