Ensure window border icons repaint when DWM is enabled

When the user movers the cursor over the minimize/maximize/close
buttons, DWM paints them in a hover state. If the user subsequently
moves the cursor out of the window, DWM was not repainting the icons
with the unhovered state because Chrome ate the WM_NCMOUSELEAVE
message.This change notifies DWM of the exit.

BUG=637114
TEST=Unmaximize Chrome. Hover Red-X at top right. Fling mouse upward.

R=sky@chromium.org

Review-Url: https://codereview.chromium.org/2244263002
Cr-Commit-Position: refs/heads/master@{#412541}

ui/views/win/hwnd_message_handler.cc

@@ -2652,8 +2652,13 @@ 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())
+        handled = DwmDefWindowProc(hwnd(), WM_NCMOUSELEAVE, 0, 0, NULL) != 0;
       break;
+    }
 
     default:
       left_button_down_on_caption_ = false;