Fixes flake timeout for DeferAllScriptRestoredPreviewWithBackForwardCache

Replaces waiting on a histogram recording to know when a navigation is
complete with a content::WaitForLoadStop(). This seems to address the
flakey timeout I was reproducing locally.

Bug: 1032558, 1035377
Change-Id: I2a6e032ea6b112988699ea06275bbd28ec261362
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974431
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726164}

chrome/browser/previews/defer_all_script_browsertest.cc

@@ -534,9 +534,7 @@ IN_PROC_BROWSER_TEST_F(DeferAllScriptBrowserTest,
   ukm::TestAutoSetUkmRecorder test_ukm_recorder;
 
   // Wait for initial page load to complete.
-  RetryForHistogramUntilCountReached(
-      &histogram_tester, "PageLoad.DocumentTiming.NavigationToLoadEventFired",
-      1);
+  content::WaitForLoadStop(web_contents());
 
   // Navigate to DeferAllScript url expecting a DeferAllScript preview.
   ui_test_utils::NavigateToURL(browser(), url);
@@ -613,9 +611,7 @@ IN_PROC_BROWSER_TEST_F(
   ukm::TestAutoSetUkmRecorder test_ukm_recorder;
 
   // Wait for initial page load to complete.
-  RetryForHistogramUntilCountReached(
-      &histogram_tester, "PageLoad.DocumentTiming.NavigationToLoadEventFired",
-      1);
+  content::WaitForLoadStop(web_contents());
 
   // Adjust the network triggering condition to not choose preview for this
   // navigation.