List all pending tasks in the ~TestBrowserThreadBundle CHECK

The current check fail message for ~TestBrowserThreadBundle can be augmented
by listing all pending tasks. This should make it easier to diagnose flakes.

I've seen this happen in ParentAccessServiceTest.NoValidConfigAvailable

Bug: 935515
Change-Id: If88ce656091ba4176cfdbf2c7313a7861b64379a
Reviewed-on: https://chromium-review.googlesource.com/c/1486352Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635605}

content/public/test/test_browser_thread_bundle.cc

@@ -54,7 +54,7 @@ TestBrowserThreadBundle::~TestBrowserThreadBundle() {
     // blocked upon it could make a test flaky whereas by flushing we guarantee
     // it will blow up).
     RunAllTasksUntilIdle();
-    CHECK(MainThreadIsIdle());
+    CHECK(MainThreadIsIdle()) << sequence_manager()->DescribeAllPendingTasks();
   }
 
   BrowserTaskExecutor::ResetForTesting();