AppCache: Use a BrowserTaskEnvironment in MockAppCacheStorageTest.

This CL replaces the test's base::test::TaskEnvironment with a
content::BrowserTaskEnvironment.

The QuotaClient mojofication benefits from having this test use a
content::BrowserTaskEnvironment, which can supply a TaskRunner for the
IO thread.

Bug: 1016065
Change-Id: Ia5cbe1a1ecafc6fd0d37972c28a44dfb5bc2d0ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519884
Commit-Queue: Victor Costan <pwnall@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824215}

content/browser/appcache/mock_appcache_storage_unittest.cc

@@ -6,12 +6,12 @@
 #include <stdint.h>
 
 #include "base/run_loop.h"
-#include "base/test/task_environment.h"
 #include "content/browser/appcache/appcache.h"
 #include "content/browser/appcache/appcache_group.h"
 #include "content/browser/appcache/appcache_response_info.h"
 #include "content/browser/appcache/appcache_storage.h"
 #include "content/browser/appcache/mock_appcache_service.h"
+#include "content/public/test/browser_task_environment.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/blink/public/mojom/appcache/appcache_info.mojom.h"
 
@@ -85,7 +85,7 @@ class MockAppCacheStorageTest : public testing::Test {
   };
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  content::BrowserTaskEnvironment task_environment_;
 };
 
 TEST_F(MockAppCacheStorageTest, LoadCache_Miss) {