cocoa: Don't use Chrome's UnderlayOpenGLHostingWindow.

The UnderlayOpenGLHostingWindow would add a semi-transparent layer under
the window, I'm not very sure what does this class exactly do, but
removing it seems no harm to the renderer.

Fixes atom/atom#877.

browser/atom_event_processing_window.h

@@ -8,12 +8,11 @@
 #import <Cocoa/Cocoa.h>
 
 #include "base/memory/scoped_nsobject.h"
-#import "ui/base/cocoa/underlay_opengl_hosting_window.h"
 
 // Override NSWindow to access unhandled keyboard events (for command
 // processing); subclassing NSWindow is the only method to do
 // this.
-@interface AtomEventProcessingWindow : UnderlayOpenGLHostingWindow {
+@interface AtomEventProcessingWindow : NSWindow {
  @private
   BOOL redispatchingEvent_;
   BOOL eventHandled_;