[Mac] Activate notifications on mouse *up*.

Buttons respond on mouse up. This is nice because it gives the user a
chance to change their mind by moving the mouse away from the button
before releasing.

This makes notifications behave the same way.

BUG=653862

Review-Url: https://codereview.chromium.org/2400033003
Cr-Commit-Position: refs/heads/master@{#423877}

ui/message_center/cocoa/notification_controller.mm

@@ -184,12 +184,15 @@
   return self;
 }
 
-- (void)mouseDown:(NSEvent*)event {
-  if ([event type] != NSLeftMouseDown) {
-    [super mouseDown:event];
+- (void)mouseUp:(NSEvent*)event {
+  if (event.type != NSLeftMouseUp) {
+    [super mouseUp:event];
     return;
   }
-  [controller_ notificationClicked];
+  if (NSPointInRect([self convertPoint:event.locationInWindow fromView:nil],
+                    self.bounds)) {
+    [controller_ notificationClicked];
+  }
 }
 
 - (NSView*)hitTest:(NSPoint)point {