Add instancetype return to test helper (#393)

Author: mdmathias

GoogleSignIn/Tests/Unit/GIDFakeFetcherService.h

@@ -26,7 +26,7 @@
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
 /// Creates an instance of this fake with an authorizer.
-- initWithAuthorizer:(id<GTMFetcherAuthorizationProtocol>)authorizer;
+- (instancetype)initWithAuthorizer:(id<GTMFetcherAuthorizationProtocol>)authorizer;
 #pragma clang diagnostic pop
 
 /// Returns the list of |GPPFakeFetcher| objects that have been created.