ScrollableArea::pixelsPerLine should return viewport pixels in use-zoom-for-dsf mode

third_party/WebKit/Source/platform/HostWindow.h

Index: third_party/WebKit/Source/platform/HostWindow.h
diff --git a/third_party/WebKit/Source/platform/HostWindow.h b/third_party/WebKit/Source/platform/HostWindow.h
index f3b5934d626f33f76d932879fd14e933f406676c..7462ec6d661567a5e051bfbd7e61da6553a169b5 100644
--- a/third_party/WebKit/Source/platform/HostWindow.h
+++ b/third_party/WebKit/Source/platform/HostWindow.h
@@ -50,8 +50,8 @@ public:
     // Converts the rect from the viewport coordinates to screen coordinates.
     virtual IntRect viewportToScreen(const IntRect&) const = 0;
 
-    // Converts the rect from the window coordinates to the viewport coordinates.
-    virtual FloatRect windowToViewport(const FloatRect&) const = 0;
+    // Converts the scalar value from the window coordinates to the viewport scale.

[skobes, 2016/02/16 18:03:12]

Is it documented anywhere what window and viewport coordinates are?  If I
understand our chat discussion correctly it is

  window coordinates == DIPs
  viewport coordinates == (use-zoom-for-dsf ? physical pixels : DIPs)

but this was not at all clear to me before.  It would be good for the comment to
explain this or link to a doc.

+    virtual float windowToViewportScalar(const float) const = 0;
 
     virtual void scheduleAnimation(Widget*) = 0;
 };