Define push_messaging module with PushManager skeleton.

Source/modules/modules.gypi

 {
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
     # ways.
     'extra_blink_module_idl_files': [],
     'extra_blink_module_files': [],
     # Files for which bindings (.cpp and .h files) will be generated
@@ skipping 88 matching lines @@
       'mediastream/RTCPeerConnection.idl',
       'mediastream/RTCSessionDescription.idl',
       'mediastream/RTCSessionDescriptionCallback.idl',
       'mediastream/RTCStatsCallback.idl',
       'mediastream/RTCStatsReport.idl',
       'mediastream/RTCStatsResponse.idl',
       'mediastream/SourceInfo.idl',
       'notifications/Notification.idl',
       'notifications/NotificationPermissionCallback.idl',
       'performance/WorkerPerformance.idl',
+      'push_messaging/PushManager.idl',
       'quota/DeprecatedStorageInfo.idl',
       'quota/DeprecatedStorageQuota.idl',
       'quota/StorageErrorCallback.idl',
       'quota/StorageInfo.idl',
       'quota/StorageQuota.idl',
       'quota/StorageQuotaCallback.idl',
       'quota/StorageUsageCallback.idl',
       'serviceworkers/Cache.idl',
       'serviceworkers/FetchEvent.idl',
       'serviceworkers/InstallEvent.idl',
@@ skipping 87 matching lines @@
       'indexeddb/WindowIndexedDatabase.idl',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.idl',
       'mediasource/HTMLVideoElementMediaSource.idl',
       'mediasource/URLMediaSource.idl',
       'mediasource/WindowMediaSource.idl',
       'mediastream/NavigatorMediaStream.idl',
       'mediastream/URLMediaStream.idl',
       'navigatorcontentutils/NavigatorContentUtils.idl',
       'performance/SharedWorkerPerformance.idl',
       'performance/WorkerGlobalScopePerformance.idl',
+      'push_messaging/NavigatorPushManager.idl',
       'quota/NavigatorStorageQuota.idl',
       'quota/WindowQuota.idl',
       'quota/WorkerNavigatorStorageQuota.idl',
       'screen_orientation/ScreenOrientation.idl',
       'serviceworkers/NavigatorServiceWorker.idl',
       'speech/WindowSpeechSynthesis.idl',
       'vibration/NavigatorVibration.idl',
       'webdatabase/WindowWebDatabase.idl',
       'webdatabase/WorkerGlobalScopeWebDatabase.idl',
       'webmidi/NavigatorWebMIDI.idl',
@@ skipping 308 matching lines @@
       'notifications/Notification.h',
       'notifications/NotificationClient.h',
       'notifications/NotificationController.cpp',
       'notifications/NotificationController.h',
       'notifications/NotificationPermissionCallback.h',
       'performance/SharedWorkerPerformance.cpp',
       'performance/WorkerGlobalScopePerformance.cpp',
       'performance/WorkerGlobalScopePerformance.h',
       'performance/WorkerPerformance.cpp',
       'performance/WorkerPerformance.h',
+      'push_messaging/NavigatorPushManager.cpp',
+      'push_messaging/NavigatorPushManager.h',
+      'push_messaging/PushManager.cpp',
+      'push_messaging/PushManager.h',
       'quota/DeprecatedStorageInfo.cpp',
       'quota/DeprecatedStorageInfo.h',
       'quota/DeprecatedStorageQuota.cpp',
       'quota/DeprecatedStorageQuota.h',
       'quota/DeprecatedStorageQuotaCallbacksImpl.cpp',
       'quota/DeprecatedStorageQuotaCallbacksImpl.h',
       'quota/DOMWindowQuota.cpp',
       'quota/DOMWindowQuota.h',
       'quota/NavigatorStorageQuota.cpp',
       'quota/NavigatorStorageQuota.h',
@@ skipping 327 matching lines @@
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
     ],
   },
 }