Deterministic script ordering for createAdFrameWithWindowStopAbortedLoad

The scripts added by createAdFrameWithWindowStopAbortedLoad must have
async set to false to ensure that they are loaded in insertion order.
This allows the last script's onload handler signal that all three
scripts have loaded.

The current behavior -- with non-deterministic ordering -- is believed
to be the cause of a flaky test (AdTaggingBrowserTest.
ChildrenOfFrameWithWindowStopAbortedLoad_StillCorrectlyTagged)

Bug: 1069346
Change-Id: I854eeee46c924920b9842701a6256a119cf676ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144832Reviewed-by: John Delaney <johnidel@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758213}

chrome/test/data/ad_tagging/ad_script.js

@@ -65,16 +65,21 @@ function createAdFrameWithWindowStopAbortedLoad(name) {
   document.body.appendChild(frame);
   frame.contentWindow.stop();
 
-  // We load the scripts in frame_factory.html to allow subframe creation.
+  // We load the scripts in frame_factory.html to allow subframe creation. We
+  // set the async attribute to false to ensure that these scripts are loaded in
+  // insertion order.
   let script1 = document.createElement('script');
+  script1.async = false;
   script1.src = 'create_frame.js';
   frame.contentDocument.head.appendChild(script1);
 
   let script2 = document.createElement('script');
+  script2.async = false;
   script2.src = 'ad_script.js';
   frame.contentDocument.head.appendChild(script2);
 
   let script3 = document.createElement('script');
+  script3.async = false;
   script3.src = 'ad_script_2.js';
   // Set title so we know when all scripts have loaded.
   script3.onload = function() {