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

third_party/WebKit/public/web/WebView.h

 /*
  * Copyright (C) 2009, 2010, 2011, 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
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  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 275 matching lines @@
 
     // Sets the display mode of the web app.
     virtual void setDisplayMode(WebDisplayMode) = 0;
 
     // The ratio of the current device's screen DPI to the target device's screen DPI.
     virtual float deviceScaleFactor() const = 0;
 
     // Sets the ratio as computed by computePageScaleConstraints.
     virtual void setDeviceScaleFactor(float) = 0;
 
+    // Sets the additional zoom factor used for device scale factor.

[Rick Byers, 2015/10/30 17:50:10]

Can you please elaborate in this (and/or above) comments?  At the moment I'm
confused by the distinction of the three values (though I admit it's been awhile
since I read your design doc).

The plan is to remove deviceScaleFactor from blink (so this API will replace the
above one), right?  Adding a comment to clarify that we're in transition should
help.

[oshima, 2015/10/30 18:14:18]

So the idea is to use zoom *mechanism* to implement DSF behavior, but we still
need to keep
zooming semantics as a browser feature (one you can set 120% 150% using
ctrl/+-).

Therefore, we have two inputs:

zoom level
DSF

and the actual zoom factor applied inside Blink is:

DSF * zoom factor (computed from zoom level)
I updated the comment.

[Rick Byers, 2015/10/30 19:03:21]

Makes sense, thanks!
WebKit/public/ LGTM

+    virtual void setZoomFactorForDeviceScaleFactor(float) = 0;
+
     // Set and reset the device color profile.
     virtual void setDeviceColorProfile(const WebVector<char>&) = 0;
     virtual void resetDeviceColorProfile() = 0;
 
     // Auto-Resize -----------------------------------------------------------
 
     // In auto-resize mode, the view is automatically adjusted to fit the html
     // content within the given bounds.
     virtual void enableAutoResizeMode(
         const WebSize& minSize,
@@ skipping 174 matching lines @@
     // context's ability to deal with that failure gracefully can be tested.
     virtual void forceNextDrawingBufferCreationToFail() = 0;
 
 protected:
     ~WebView() {}
 };
 
 } // namespace blink
 
 #endif