Remove SyncTaskToken reboot and re-enable disabled SyncFileSystemTest

This CL removes auto-restart of SyncTaskManager. It used to restart
a chain of asynchronous task on an unexpected chaing breakage unless
the browser is in the shutdown sequence. However, as the shutdown
detection is unstable, that causes a restart loop.

Bug: 750800
Change-Id: I2ca231b84908f7c015b4cd97ad02a43f423b20e4
Reviewed-on: https://chromium-review.googlesource.com/597552Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491331}

chrome/browser/extensions/api/sync_file_system/sync_file_system_browsertest.cc

@@ -147,14 +147,7 @@ class SyncFileSystemTest : public extensions::PlatformAppBrowserTest,
   DISALLOW_COPY_AND_ASSIGN(SyncFileSystemTest);
 };
 
-#if defined(OS_CHROMEOS)
-// Flaky on ChromiumOS, http://crbug.com/750800.
-#define MAYBE_AuthorizationTest DISABLED_AuthorizationTest
-#else
-#define MAYBE_AuthorizationTest AuthorizationTest
-#endif
-
-IN_PROC_BROWSER_TEST_F(SyncFileSystemTest, MAYBE_AuthorizationTest) {
+IN_PROC_BROWSER_TEST_F(SyncFileSystemTest, AuthorizationTest) {
   ExtensionTestMessageListener open_failure(
       "checkpoint: Failed to get syncfs", true);
   ExtensionTestMessageListener bar_created(

chrome/browser/sync_file_system/drive_backend/sync_task_token.cc

@@ -59,25 +59,7 @@ void SyncTaskToken::UpdateTask(const tracked_objects::Location& location,
   DVLOG(2) << "Token updated: " << location_.ToString();
 }
 
-SyncTaskToken::~SyncTaskToken() {
-  // All task on Client must hold TaskToken instance to ensure
-  // no other tasks are running. Also, as soon as a task finishes to work,
-  // it must return the token to TaskManager.
-  // Destroying a token with valid |client| indicates the token was
-  // dropped by a task without returning.
-  if (task_runner_.get() && task_runner_->RunsTasksInCurrentSequence() &&
-      manager_ && manager_->IsRunningTask(token_id_)) {
-    NOTREACHED()
-        << "Unexpected TaskToken deletion from: " << location_.ToString();
-
-    // Reinitializes the token.
-    SyncTaskManager::NotifyTaskDone(
-        base::WrapUnique(new SyncTaskToken(manager_, task_runner_.get(),
-                                           token_id_, std::move(task_blocker_),
-                                           SyncStatusCallback())),
-        SYNC_STATUS_OK);
-  }
-}
+SyncTaskToken::~SyncTaskToken() {}
 
 // static
 SyncStatusCallback SyncTaskToken::WrapToCallback(