Updating Blink-side WebGamepad interface

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 29 matching lines @@
       'filesystem/FileCallback.idl',
       'filesystem/FileEntry.idl',
       'filesystem/FileEntrySync.idl',
       'filesystem/FileSystemCallback.idl',
       'filesystem/FileWriter.idl',
       'filesystem/FileWriterCallback.idl',
       'filesystem/FileWriterSync.idl',
       'filesystem/Metadata.idl',
       'filesystem/MetadataCallback.idl',
       'gamepad/Gamepad.idl',
+      'gamepad/GamepadButton.idl',
       'gamepad/GamepadList.idl',
       'geolocation/Coordinates.idl',
       'geolocation/Geolocation.idl',
       'geolocation/Geoposition.idl',
       'geolocation/PositionCallback.idl',
       'geolocation/PositionError.idl',
       'geolocation/PositionErrorCallback.idl',
       'indexeddb/IDBCursor.idl',
       'indexeddb/IDBCursorWithValue.idl',
       'indexeddb/IDBDatabase.idl',
@@ skipping 273 matching lines @@
       'filesystem/InspectorFrontendHostFileSystem.h',
       'filesystem/LocalFileSystem.cpp',
       'filesystem/LocalFileSystem.h',
       'filesystem/Metadata.h',
       'filesystem/MetadataCallback.h',
       'filesystem/SyncCallbackHelper.h',
       'filesystem/WorkerGlobalScopeFileSystem.cpp',
       'filesystem/WorkerGlobalScopeFileSystem.h',
       'gamepad/Gamepad.cpp',
       'gamepad/Gamepad.h',
+      'gamepad/GamepadButton.cpp',
+      'gamepad/GamepadButton.h',
       'gamepad/GamepadList.cpp',
       'gamepad/GamepadList.h',
       'gamepad/NavigatorGamepad.cpp',
       'gamepad/NavigatorGamepad.h',
       'geolocation/Coordinates.cpp',
       'geolocation/Geolocation.cpp',
       'geolocation/GeolocationController.cpp',
       'geolocation/GeolocationInspectorAgent.cpp',
       'geolocation/Geoposition.cpp',
       'geolocation/NavigatorGeolocation.cpp',
@@ skipping 490 matching lines @@
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
     ],
   },
 }