[Media History] Deflake browser test

Deflake the browser test by using wait for db.

BUG=1052605,1058563

Change-Id: Iba71c6a5b6c1145f777d028ded4638b404434b44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122614Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753759}

chrome/browser/media/history/media_history_browsertest.cc

@@ -276,7 +276,7 @@ class MediaHistoryBrowserTest : public InProcessBrowserTest,
     ui_test_utils::NavigateToURL(browser, embedded_test_server()->base_url());
 
     // Wait until the session has finished saving.
-    content::RunAllTasksUntilIdle();
+    WaitForDB(GetMediaHistoryService(browser));
   }
 
   const GURL GetTestURL() const {
@@ -301,6 +301,12 @@ class MediaHistoryBrowserTest : public InProcessBrowserTest,
         browser->profile()->GetOffTheRecordProfile());
   }
 
+  static void WaitForDB(MediaHistoryKeyedService* service) {
+    base::RunLoop run_loop;
+    service->PostTaskToDBForTest(run_loop.QuitClosure());
+    run_loop.Run();
+  }
+
   Browser* CreateBrowserFromParam() {
     if (GetParam() == TestState::kIncognito) {
       return CreateIncognitoBrowser();