Added plumbing for the availablechange event from Blink to WebPresentationClient.

Source/modules/modules.gypi

Index: Source/modules/modules.gypi
diff --git a/Source/modules/modules.gypi b/Source/modules/modules.gypi
index eda88642c6c648be6deabe5a7937d7bb51a0300d..6707871e2ce9a7a774f5afc447773f1004410f9d 100644
--- a/Source/modules/modules.gypi
+++ b/Source/modules/modules.gypi
@@ -125,6 +125,7 @@
       'performance/WorkerPerformance.idl',
       'permissions/Permissions.idl',
       'permissions/PermissionStatus.idl',
+      'presentation/AvailableChangeEvent.idl',
       'presentation/Presentation.idl',
       'presentation/PresentationSession.idl',
       'push_messaging/PushEvent.idl',
@@ -288,6 +289,7 @@
       'mediastream/RTCIceCandidateEvent.idl',
       'navigatorconnect/CrossOriginConnectEvent.idl',
       'notifications/NotificationEvent.idl',
+      'presentation/AvailableChangeEvent.idl',
       'push_messaging/PushEvent.idl',
       'serviceworkers/ExtendableEvent.idl',
       'serviceworkers/FetchEvent.idl',
@@ -322,6 +324,7 @@
       'mediastream/RTCSessionDescriptionInit.idl',
       'notifications/NotificationEventInit.idl',
       'notifications/NotificationOptions.idl',
+      'presentation/AvailableChangeEventInit.idl',
       'push_messaging/PushEventInit.idl',
       'serviceworkers/CacheQueryOptions.idl',
       'serviceworkers/ExtendableEventInit.idl',
@@ -387,6 +390,8 @@
       '<(blink_modules_output_dir)/notifications/NotificationEventInit.h',
       '<(blink_modules_output_dir)/notifications/NotificationOptions.cpp',
       '<(blink_modules_output_dir)/notifications/NotificationOptions.h',
+      '<(blink_modules_output_dir)/presentation/AvailableChangeEventInit.cpp',
+      '<(blink_modules_output_dir)/presentation/AvailableChangeEventInit.h',
       '<(blink_modules_output_dir)/push_messaging/PushEventInit.cpp',
       '<(blink_modules_output_dir)/push_messaging/PushEventInit.h',
       '<(blink_modules_output_dir)/serviceworkers/CacheQueryOptions.cpp',
@@ -861,10 +866,14 @@
       'permissions/Permissions.h',
       'permissions/PermissionStatus.cpp',
       'permissions/PermissionStatus.h',
+      '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',
       'push_messaging/PushController.cpp',