Re-enable VolumeManagerTest.OnFormatEvent_CompletedFailed

Mitigate this issue: make the thread_bundle RunUntilIdle when tearing
down these unittests. See crbug.com/943570#c7 for rationale.

No-try: true
Bug: 943570
Change-Id: Ib8df70054ea9de3d7277da0b49e8885bf103b44a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530385Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642405}

chrome/browser/chromeos/file_manager/volume_manager_unittest.cc

@@ -254,6 +254,7 @@ class VolumeManagerTest : public testing::Test {
     main_profile_.reset();
     disk_mount_manager_.reset();
     chromeos::PowerManagerClient::Shutdown();
+    thread_bundle_.RunUntilIdle();
   }
 
   Profile* profile() const { return main_profile_->profile(); }
@@ -726,8 +727,7 @@ TEST_F(VolumeManagerTest, OnFormatEvent_Completed) {
   volume_manager()->RemoveObserver(&observer);
 }
 
-// TODO(crbug.com/943570): test is flaky, disabled.
-TEST_F(VolumeManagerTest, DISABLED_OnFormatEvent_CompletedFailed) {
+TEST_F(VolumeManagerTest, OnFormatEvent_CompletedFailed) {
   LoggingObserver observer;
   volume_manager()->AddObserver(&observer);