Pass device scale factor from display to ws.

services/ui/display/platform_screen_delegate.h

Index: services/ui/display/platform_screen_delegate.h
diff --git a/services/ui/display/platform_screen_delegate.h b/services/ui/display/platform_screen_delegate.h
index 9c0e144a37778991d893b045047c09a01d381cd5..46988d2009bca215da9377741fba50dc439d7374 100644
--- a/services/ui/display/platform_screen_delegate.h
+++ b/services/ui/display/platform_screen_delegate.h
@@ -9,6 +9,7 @@
 
 namespace gfx {
 class Rect;
+class Size;
 }
 
 namespace display {
@@ -19,17 +20,20 @@ class PlatformScreen;
 // and/or virtual displays by PlatformScreen.
 class PlatformScreenDelegate {
  public:
-  // TODO(kylechar): We need to provide more than just the window bounds when
-  // displays are added or modified.
-
   // Called when a display is added. |bounds| is in DIP.
-  virtual void OnDisplayAdded(int64_t id, const gfx::Rect& bounds) = 0;
+  virtual void OnDisplayAdded(int64_t id,
+                              const gfx::Rect& bounds,
+                              const gfx::Size& pixel_size,

[rjkroege, 2016/09/21 13:23:46]

and pixel_size is not in dip? i.e. in DDP? Please be more explicit about this. 
And while your at that, make sure that you say that id is a display id?

[kylechar, 2016/09/21 16:31:12]

Done.

+                              float scale_factor) = 0;

[rjkroege, 2016/09/21 13:23:46]

If we have the size in DP and the size in DIP, then the scale_factor is useful?

[kylechar, 2016/09/21 16:31:12]

Usually, DIP = DDP * (ui_scale_factor / device_scale_factor). In that case we
need at least three of the four values to compute the last. I'm just assuming
ui_scale_factor is 1.0 for now. ash::DisplayManager has all the code that
supports ui_scale_factor (which I don't want to reimplement).

Except there is a special case where ui_scale_factor=0.8 and
device_scale_factor=1.25 so that formula doesn't work. So you probably need all
four values?

 
   // Called when a display is removed.
   virtual void OnDisplayRemoved(int64_t id) = 0;
 
   // Called when a display is modified. |bounds| is in DIP.
-  virtual void OnDisplayModified(int64_t id, const gfx::Rect& bounds) = 0;
+  virtual void OnDisplayModified(int64_t id,
+                                 const gfx::Rect& bounds,
+                                 const gfx::Size& pixel_size,
+                                 float scale_factor) = 0;
 
  protected:
   virtual ~PlatformScreenDelegate() {}