Create ScreenWin::GetSystemMetricsInDIP and Correctly Deprecate display::win::GetSystemMetricsInDIP

ui/display/win/screen_win.h

Index: ui/display/win/screen_win.h
diff --git a/ui/display/win/screen_win.h b/ui/display/win/screen_win.h
index 0a16da017ce1bfb0afe581c5623f185c1bdf4477..23792dec09f9cf48fd5ac3133eb02baa1c07f97b 100644
--- a/ui/display/win/screen_win.h
+++ b/ui/display/win/screen_win.h
@@ -92,6 +92,9 @@ class DISPLAY_EXPORT ScreenWin : public display::Screen {
   // Returns the result of GetSystemMetrics for |metric| scaled to |hwnd|'s DPI.
   static int GetSystemMetricsForHwnd(HWND hwnd, int metric);
 
+  // Returns the result of GetSystemMetrics for |metric| in DIP.

[sky, 2016/07/27 21:44:11]

When would you use this rather than supplying an HWND and using it's scale?

[robliao, 2016/07/27 22:04:01]

You'd use this when you actually want to just do things in DIP. That turns out
to be HWND invariant.

GetSystemMetrics always returns results in physical pixels based off of the
primary monitor's DPI. If we removed GetSystemMetricsInDIP, to obtain the
equivalent result, you'd perform this calculation:
GetSystemMetricsForHwnd(hwnd, ...) / GetScaleFactorForHwnd(hwnd). This is
equivalent to

(GetSystemMetrics(metric) * HwndScaleFactor / PrimaryMonitorScaleFactor) /
HwndScaleFactor, leaving us with
GetSystemMetrics(metric) / PrimaryMonitorScaleFactor

Developers who need a physical metric for an HWND (like
BrowserDesktopWindowTreeHostWin's physical frame size) will want use
GetSystemMetricsForHwnd and avoid any rounding errors that arise from
GetSystemMetrics.

I've amended the comments to capture some of this.

+  static int GetSystemMetricsInDIP(int metric);
+
   // Returns |hwnd|'s scale factor.
   static float GetScaleFactorForHWND(HWND hwnd);