[Mac] Move the large block of NSEvent forward declarations to a shared header.

base/base.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
     },
     'target_conditions': [
@@ skipping 239 matching lines @@
           'mac/scoped_ioplugininterface.h',
           'mac/scoped_launch_data.h',
           'mac/scoped_mach_port.cc',
           'mac/scoped_mach_port.h',
           'mac/scoped_nsautorelease_pool.h',
           'mac/scoped_nsautorelease_pool.mm',
           'mac/scoped_nsexception_enabler.h',
           'mac/scoped_nsexception_enabler.mm',
           'mac/scoped_sending_event.h',
           'mac/scoped_sending_event.mm',
+          'mac/sdk_forward_declarations.h',
           'memory/aligned_memory.cc',
           'memory/aligned_memory.h',
           'memory/discardable_memory.cc',
           'memory/discardable_memory.h',
           'memory/discardable_memory_android.cc',
           'memory/discardable_memory_mac.cc',
           'memory/linked_ptr.h',
           'memory/manual_constructor.h',
           'memory/raw_scoped_refptr_mismatch_checker.h',
           'memory/ref_counted.cc',
@@ skipping 549 matching lines @@
           ['<(use_system_nspr)==1 and >(nacl_untrusted_build)==0', {
             'sources/': [
               ['exclude', '^third_party/nspr/'],
             ],
           }],
         ],
       }],
     ],
   },
 }