[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_service_unittest.cc

 // Copyright 2017 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 <map>
 #include <memory>
 #include <utility>
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
@@ skipping 122 matching lines @@
 
     service_ = base::MakeUnique<BackgroundFetchServiceImpl>(
         0 /* render_process_id */, context_);
   }
 
   void TearDown() override {
     BackgroundFetchTestBase::TearDown();
 
     service_.reset();
 
-    context_->Shutdown();
     context_ = nullptr;
 
     // Give pending shutdown operations a chance to finish.
     base::RunLoop().RunUntilIdle();
   }
 
  private:
   void DidGetRegistration(
       base::Closure quit_closure,
       blink::mojom::BackgroundFetchError* out_error,
@@ skipping 550 matching lines @@
     const bool has_alternative_tag =
         tags[0] == kAlternativeTag || tags[1] == kAlternativeTag;
 
     EXPECT_TRUE(has_example_tag);
     EXPECT_TRUE(has_alternative_tag);
   }
 }
 
 }  // namespace
 }  // namespace content