Ensure window border icons repaint when DWM is enabled

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 9b40b5089d88dca57f1a446cb805ed2431a6d7e1..72854ea9d39dde94b7abe49c5848001ba1c04dce 100644
--- a/ui/views/win/hwnd_message_handler.cc
+++ b/ui/views/win/hwnd_message_handler.cc
@@ -2638,8 +2638,15 @@ bool HWNDMessageHandler::HandleMouseInputForCaption(unsigned int message,
       break;
     }
 
-    case WM_NCMOUSELEAVE:
+    case WM_NCMOUSELEAVE: {
+      // If the DWM is rendering the window controls, we need to give the DWM's
+      // default window procedure the chance to repaint the window border icons
+      if (HasSystemFrame()) {

[sky, 2016/08/15 16:17:13]

Does the conditional really matter here?

[elawrence, 2016/08/15 16:31:14]

I'm afraid I'm ill-qualified to answer that; I matched the guard around the
other call to DwmDefWindowProc earlier in this file. 

I can say that without my change, Windows 7 shows the bug when an Aero theme is
selected, regardless of whether transparent glass is on. The bug does not exist
in the Windows 7 "Basic" theme. 

After my patch is applied, the bug is not present when an Aero theme is selected
(nor when Basic is chosen).

[msw, 2016/08/15 18:57:35]

The issue doesn't seem to repro with non-glass browser frames (ie. theme
applied) on Win7 Aero Glass, so it's probably not necessary to call
DwmDefWindowProc if HasSystemFrame, but it probably wouldn't hurt either.

+        LRESULT result;
+        DwmDefWindowProc(hwnd(), WM_NCMOUSELEAVE, 0, 0, &result);

[msw, 2016/08/15 18:57:35]

nit q: I wonder if we should set handled to true for certain result values? That
might prevent some redundant dispatching later, but it's probably not critical.
Otherwise, can we pass null for the result if we don't care to check it?

[elawrence, 2016/08/16 16:00:49]

Done.

+      }
       break;
+    }
 
     default:
       left_button_down_on_caption_ = false;