Re-enable idleness check in ~TestBrowserThreadBundle

All known issues identified by it have been fixed :)
https://bugs.chromium.org/p/chromium/issues/list?can=1&q=TestBrowserThreadBundle+MainThreadIsIdle&x=m&y=releaseblock&cells=ids

R=alexclarke@chromium.org

Bug: 938126
Change-Id: I3aa25d9d2c65a7520926d7700e45d52fc84104d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614779
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660314}

content/public/test/test_browser_thread_bundle.cc

@@ -50,8 +50,7 @@ TestBrowserThreadBundle::~TestBrowserThreadBundle() {
   // 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.
-  // TODO(crbug.com/938126): Enable this CHECK once flaky tests have been fixed.
-  // CHECK(MainThreadIsIdle()) << sequence_manager()->DescribeAllPendingTasks();
+  CHECK(MainThreadIsIdle()) << sequence_manager()->DescribeAllPendingTasks();
 
   BrowserTaskExecutor::ResetForTesting();