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..9754f7f0bcc12842b6518cd507c46c0a5f8d7837 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;
 
+    // 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;