Add IDL and initial Blink API for Background Sync

public/platform/modules/background_sync/WebSyncProvider.h

Index: public/platform/modules/background_sync/WebSyncProvider.h
diff --git a/public/platform/modules/background_sync/WebSyncProvider.h b/public/platform/modules/background_sync/WebSyncProvider.h
index 841bfd5ad7c733c5ab73f8c63b4be1daa7e25f7b..ef19784be0b00808b36a8db17bd5c4b6f9c8bb29 100644
--- a/public/platform/modules/background_sync/WebSyncProvider.h
+++ b/public/platform/modules/background_sync/WebSyncProvider.h
@@ -7,6 +7,7 @@
 
 #include "public/platform/WebCallbacks.h"
 #include "public/platform/WebCommon.h"
+#include "public/platform/WebString.h"
 
 namespace blink {
 
@@ -16,14 +17,17 @@ struct WebSyncRegistration;
 struct WebSyncRegistrationOptions;
 
 using WebSyncRegistrationCallbacks = WebCallbacks<WebSyncRegistration, WebSyncError>;
+using WebSyncUnregistrationCallbacks = WebCallbacks<bool, WebSyncError>;
 
 class WebSyncProvider {
 public:
     virtual ~WebSyncProvider() { }
 
     // Takes ownership of the WebSyncRegistrationCallbacks.
-    // Does not take ownership of the WebServiceWorkerRegistration.
     virtual void registerBackgroundSync(const WebSyncRegistrationOptions*, WebSyncRegistrationCallbacks*) = 0;
+
+    // Takes ownership of the WebSyncRegistrationCallbacks.
+    virtual void unregisterBackgroundSync(const WebString&, WebSyncUnregistrationCallbacks*) = 0;
 };
 
 } // namespace blink