Factor out GamepadCommon base class from Gamepad and WebKitGamepad

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 351 matching lines @@
       '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/GamepadCommon.cpp',
+      'gamepad/GamepadCommon.h',
       'gamepad/GamepadList.cpp',
       'gamepad/GamepadList.h',
       'gamepad/NavigatorGamepad.cpp',
       'gamepad/NavigatorGamepad.h',
       'gamepad/WebKitGamepad.cpp',
       'gamepad/WebKitGamepad.h',
       'gamepad/WebKitGamepadList.cpp',
       'gamepad/WebKitGamepadList.h',
       'geolocation/Coordinates.cpp',
       'geolocation/Geolocation.cpp',
@@ skipping 495 matching lines @@
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
     ],
   },
 }