Add Plumbing for WM_DPICHANGED

ui/views/win/hwnd_message_handler.cc

Index: ui/views/win/hwnd_message_handler.cc
diff --git a/ui/views/win/hwnd_message_handler.cc b/ui/views/win/hwnd_message_handler.cc
index d8c14ee9b2776f12091f4af804e3da9a69df6ff0..1f46bee037715ff06489ff330b35ed3ee4d2e076 100644
--- a/ui/views/win/hwnd_message_handler.cc
+++ b/ui/views/win/hwnd_message_handler.cc
@@ -28,6 +28,7 @@
 #include "ui/base/win/mouse_wheel_util.h"
 #include "ui/base/win/shell.h"
 #include "ui/base/win/touch_input.h"
+#include "ui/display/win/dpi.h"
 #include "ui/events/event.h"
 #include "ui/events/event_utils.h"
 #include "ui/events/keycodes/keyboard_code_conversion_win.h"
@@ -1395,6 +1396,17 @@ LRESULT HWNDMessageHandler::OnDwmCompositionChanged(UINT msg,
   return 0;
 }
 
+LRESULT HWNDMessageHandler::OnDpiChanged(UINT msg,
+                                         WPARAM w_param,
+                                         LPARAM l_param) {
+  // We assume square-style scaling factors.
+  DCHECK_EQ(LOWORD(w_param), HIWORD(w_param));

[sky, 2016/06/27 19:09:22]

Is this DCHECK really worthwhile? Presumably folks can still run on non-square
scales and would then hit this, right? What good would the DCHECK do?

[robliao, 2016/06/27 19:40:34]

The DCHECK is only worthwhile to the extent that this is one of the leading
edges of accepting scale factor information in Chrome. The other is when we
construct the displays, which would also trigger.

I can remove it if you want.

+  SetBoundsInternal(gfx::Rect(*reinterpret_cast<RECT*>(l_param)), false);
+  delegate_->HandleWindowScaleFactorChanged(
+      display::win::GetScalingFactorFromDPI(LOWORD(w_param)));
+  return 0;
+}
+
 void HWNDMessageHandler::OnEnterMenuLoop(BOOL from_track_popup_menu) {
   if (menu_depth_++ == 0)
     delegate_->HandleMenuLoop(true);