Backport some tests in /google_apis/gaia to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/google_apis/gaia.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=blundell@chromium.org

Bug: 891670
Change-Id: I549dadbf2e0cb990ad5f98998a2b2b9f6220caef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787107
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693628}

google_apis/gaia/oauth2_access_token_fetcher_impl_unittest.cc

@@ -115,7 +115,7 @@ class OAuth2AccessTokenFetcherImplTest : public testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   MockOAuth2AccessTokenConsumer consumer_;
   URLLoaderFactoryInterceptor url_loader_factory_interceptor_;
   network::TestURLLoaderFactory url_loader_factory_;

google_apis/gaia/oauth2_api_call_flow_unittest.cc

@@ -100,7 +100,7 @@ class OAuth2ApiCallFlowTest : public testing::Test {
     AddFetchResult(url, succeeds, status, std::string());
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   network::TestURLLoaderFactory test_url_loader_factory_;
   scoped_refptr<network::SharedURLLoaderFactory> shared_factory_;
   StrictMock<MockApiCallFlow> flow_;