Future-proof against some upcoming fetcher changes.

GoogleSignIn/Tests/Unit/GIDFakeFetcherService.m

@@ -63,6 +63,15 @@ - (NSURLSession *)sessionForFetcherCreation {
   return nil;
 }
 
+// TODO: replace with GTMSessionFetcherSessionCreationBlock once that has become
+// available by updating the version dependencies.
+typedef NSURLSession *_Nullable (^GIDFakeFetcherSessionCreationBlock)(
+    id<NSURLSessionDelegate> _Nullable delegate);
+- (NSURLSession *)sessionWithCreationBlock:
+    (NS_NOESCAPE GIDFakeFetcherSessionCreationBlock)creationBlock {
+  return nil;
+}
+
 - (id<NSURLSessionDelegate>)sessionDelegate {
   return nil;
 }