Fix resource loading hints flaky test

Make the test wait for resource loading hints to be available.

Change-Id: I4b0cd3d3f03c53efcd386f16c6e70d05d9ef2ed4
Bug: 863250
Reviewed-on: https://chromium-review.googlesource.com/1136838Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575011}

chrome/browser/previews/resource_loading_hints/resource_loading_hints_browsertest.cc

@@ -280,6 +280,12 @@ IN_PROC_BROWSER_TEST_F(
 
 IN_PROC_BROWSER_TEST_F(ResourceLoadingHintsBrowserTest,
                        ResourceLoadingHintsHttpsNoWhitelisted) {
+  TestOptimizationGuideServiceObserver observer;
+  AddTestOptimizationGuideServiceObserver(&observer);
+  base::RunLoop().RunUntilIdle();
+  SetResourceLoadingHintsWhitelist({});
+  observer.WaitForNotification();
+
   base::HistogramTester histogram_tester;
   // The URL is not whitelisted.
   ui_test_utils::NavigateToURL(browser(), https_url());