Create a function to allow clients of TestingProfile to wait on the...

Create a function to allow clients of TestingProfile to wait on the destruction of its HistoryBackend.

The HistoryServiceUsesTaskScheduler feature will soon be flipped to ENABLED_BY_DEFAULT.
This will make HistoryServiceBackend's task runner dispatch to the ThreadPool instead of a
dedicated base::Thread. Tests using TestingProfile need to be prepared to stay functional when that happens.

This is done as the last mile of the chrome-wide effort to coalesce as much work as possible
to the ThreadPool.

Bug: 661143
Change-Id: Iddb851d00ea64693f669bcb656829abcb2303b10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868913Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707810}

chrome/test/base/testing_profile.cc

@@ -653,6 +653,23 @@ void TestingProfile::CreateWebDataService() {
       this, base::BindRepeating(&BuildWebDataService));
 }
 
+void TestingProfile::BlockUntilHistoryBackendDestroyed() {
+  // Only get the history service if it actually exists since the caller of the
+  // test should explicitly call CreateHistoryService to build it.
+  history::HistoryService* history_service =
+      HistoryServiceFactory::GetForProfileWithoutCreating(this);
+
+  // Nothing to destroy
+  if (!history_service) {
+    return;
+  }
+
+  base::RunLoop run_loop;
+  history_service->SetOnBackendDestroyTask(run_loop.QuitClosure());
+  HistoryServiceFactory::ShutdownForProfile(this);
+  run_loop.Run();
+}
+
 void TestingProfile::BlockUntilHistoryIndexIsRefreshed() {
   // Only get the history service if it actually exists since the caller of the
   // test should explicitly call CreateHistoryService to build it.

chrome/test/base/testing_profile.h

@@ -250,6 +250,11 @@ class TestingProfile : public Profile {
   // This is NOT invoked from CreateHistoryService.
   void BlockUntilHistoryIndexIsRefreshed();
 
+  // Blocks until the HistoryBackend is completely destroyed. This is mostly
+  // useful to ensure the destruction tasks do not outlive this class on which
+  // they depend.
+  void BlockUntilHistoryBackendDestroyed();
+
   // Allow setting a profile as Guest after-the-fact to simplify some tests.
   void SetGuestSession(bool guest);