Revert "Run pending tasks in ~TestBrowserThreadBundle."

This reverts commit 1c3948d8.

Reason for revert: The patch caused multiple content_unittest flaky.
https://bugs.chromium.org/p/chromium/issues/detail?id=938126
This CL could be relanded once these flaky tests are fixed or disabled.

Original change's description:
> Run pending tasks in ~TestBrowserThreadBundle.
> 
> This makes sure we run all remaining tasks in tests. This should be the
> case even for ExecutionMode::QUEUED as documented here:
> https://cs.chromium.org/chromium/src/base/test/scoped_task_environment.h?l=42
> 
> Also lift the restriction to only flush tasks in !MOCK_TIME && !QUEUED modes
> which is unnecessary now that TestBrowserThreadBundle is-a
> ScopedTaskEnvironment (http://crrev.com/616622).
> 
> Bug: 708584
> Change-Id: I52773e27d3875bd246bb1cd7eea92c9e11fc70bf
> Reviewed-on: https://chromium-review.googlesource.com/c/1480460
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#637288}

TBR=gab@chromium.org,knollr@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 708584
Bug: 938126
Change-Id: I8b43a5c90b8f28eee2817a94cd556b2742e2f58c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503993Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637760}

content/public/test/test_browser_thread_bundle.cc

@@ -31,27 +31,32 @@
 namespace content {
 
 TestBrowserThreadBundle::~TestBrowserThreadBundle() {
-  // This is required to ensure we run all remaining MessageLoop and
-  // TaskScheduler tasks in an atomic step. This is a bit different than
-  // production where the main thread is not flushed after it's done running
-  // but this approach is preferred in unit tests as running more tasks can
-  // merely uncover more issues (e.g. if a bad tasks is posted but never
-  // blocked upon it could make a test flaky whereas by flushing we guarantee
-  // it will blow up).
-  RunUntilIdle();
-
-  // When REAL_IO_THREAD, we need to stop the IO thread explicitly and flush
-  // again.
-  if (real_io_thread_) {
-    io_thread_->Stop();
-    RunUntilIdle();
+  // To ensure a clean teardown, each thread's message loop must be flushed
+  // just before the thread is destroyed. But stopping a fake thread does not
+  // automatically flush the message loop, so we have to do it manually.
+  // See http://crbug.com/247525 for discussion.
+  base::RunLoop().RunUntilIdle();
+  io_thread_->Stop();
+  base::RunLoop().RunUntilIdle();
+  ui_thread_->Stop();
+  base::RunLoop().RunUntilIdle();
+
+  // Skip the following steps when RunAllTasksUntilIdle might result in a hang
+  // (ExecutionMode::QUEUED) or for MainThreadType::MOCK_TIME where we haven't
+  // enforced there being no pending tasks.
+  if (main_thread_type() != MainThreadType::MOCK_TIME &&
+      execution_control_mode() != ExecutionMode::QUEUED) {
+    // This is required to ensure we run all remaining MessageLoop and
+    // TaskScheduler tasks in an atomic step. This is a bit different than
+    // production where the main thread is not flushed after it's done running
+    // but this approach is preferred in unit tests as running more tasks can
+    // merely uncover more issues (e.g. if a bad tasks is posted but never
+    // blocked upon it could make a test flaky whereas by flushing we guarantee
+    // it will blow up).
+    RunAllTasksUntilIdle();
+    CHECK(MainThreadIsIdle()) << sequence_manager()->DescribeAllPendingTasks();
   }
 
-  // The only way this check can fail after RunUntilIdle() is if a test is
-  // running its own base::Thread's. Such tests should make sure to coalesce
-  // independent threads before this point.
-  CHECK(MainThreadIsIdle()) << sequence_manager()->DescribeAllPendingTasks();
-
   BrowserTaskExecutor::ResetForTesting();
 
   // Run DestructionObservers before our fake threads go away to ensure