[PresentationAPI] Added plumbing for start/joinSession from JS to platform

Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.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
@@ skipping 862 matching lines @@
       'presentation/AvailableChangeEvent.h',
       'presentation/AvailableChangeEvent.cpp',
       'presentation/NavigatorPresentation.cpp',
       'presentation/NavigatorPresentation.h',
       'presentation/Presentation.cpp',
       'presentation/Presentation.h',
       'presentation/PresentationController.cpp',
       'presentation/PresentationController.h',
       'presentation/PresentationSession.cpp',
       'presentation/PresentationSession.h',
+      'presentation/PresentationSessionClientCallbacks.cpp',
+      'presentation/PresentationSessionClientCallbacks.h',
       'push_messaging/PushController.cpp',
       'push_messaging/PushController.h',
       'push_messaging/PushError.cpp',
       'push_messaging/PushError.h',
       'push_messaging/PushEvent.cpp',
       'push_messaging/PushEvent.h',
       'push_messaging/PushManager.cpp',
       'push_messaging/PushManager.h',
       'push_messaging/PushMessageData.cpp',
       'push_messaging/PushMessageData.h',
@@ skipping 341 matching lines @@
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'serviceworkers/CacheTest.cpp',
       'serviceworkers/ServiceWorkerContainerTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }