Annotates functions returning retained objects.

ios/chrome/browser/signin/gaia_auth_fetcher_ios_unittest.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/chrome/browser/signin/gaia_auth_fetcher_ios.h"
 
 #include <memory>
 
 #import "base/mac/scoped_nsobject.h"
 #include "base/run_loop.h"
@@ skipping 11 matching lines @@
 
 namespace {
 
 class FakeGaiaAuthFetcherIOSBridge : public GaiaAuthFetcherIOSBridge {
  public:
   FakeGaiaAuthFetcherIOSBridge(GaiaAuthFetcherIOS* fetcher,
                                web::BrowserState* browser_state)
       : GaiaAuthFetcherIOSBridge(fetcher, browser_state), mock_web_view_(nil) {}
 
  private:
-  WKWebView* CreateWKWebView() override {
+  WKWebView* CreateWKWebView() override NS_RETURNS_RETAINED {
     if (!mock_web_view_) {
       mock_web_view_ = [OCMockObject niceMockForClass:[WKWebView class]];
     }
     return [mock_web_view_ retain];
   }
   id mock_web_view_;
 };
 
 class MockGaiaConsumer : public GaiaAuthConsumer {
  public:
@@ skipping 140 matching lines @@
   [[GetMockWKWebView() expect] setNavigationDelegate:[OCMArg isNotNil]];
   [static_cast<WKWebView*>([[GetMockWKWebView() expect] andDo:^(NSInvocation*) {
     GetBridge()->URLFetchSuccess("data");
   }]) loadRequest:[OCMArg any]];
 
   gaia_auth_fetcher_->StartMergeSession("uber_token", "");
   web::BrowserState::GetActiveStateManager(&browser_state_)->SetActive(false);
   web::BrowserState::GetActiveStateManager(&browser_state_)->SetActive(true);
   EXPECT_OCMOCK_VERIFY(GetMockWKWebView());
 }