[Background Fetch] Fix race condition in Data Manager test.

The Cleanup task spawns some deletions tasks which access the cache
storage on another thread. This can cause a race condition with a
regular RunLoop, so I replaced it with the TestBrowserThreadBundle.

TBR=peter@chromium.org

Change-Id: I6ab56a4a0c72b50831461832bce6a3ab10ea5543
Reviewed-on: https://chromium-review.googlesource.com/1143404Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576487}

content/browser/background_fetch/background_fetch_data_manager_unittest.cc

@@ -1227,7 +1227,7 @@ TEST_F(BackgroundFetchDataManagerTest, Cleanup) {
 
   // Cleanup should delete the registration.
   background_fetch_data_manager_->Cleanup();
-  base::RunLoop().RunUntilIdle();
+  thread_bundle_.RunUntilIdle();
   EXPECT_EQ(0u,
             GetRegistrationUserDataByKeyPrefix(sw_id, kUserDataPrefix).size());