[IOS][UKM] Fixes chrome://ukm on iOS

Fixes the page by adding the iOS WebUI messaging script to the html.

Also modifies the JavaScript to update page content after HTML has been
completely loaded, for more robustness.

Bug: 883408
Change-Id: I43d1be5077af6d42e357bfb3a5bbcb28eb5c0d60
Reviewed-on: https://chromium-review.googlesource.com/1225300Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591126}

components/ukm/debug/ukm_internals.html

@@ -7,6 +7,10 @@
 <script src="chrome://resources/js/cr.js"></script>
 <script src="chrome://resources/js/promise_resolver.js"></script>
 <script src="chrome://resources/js/util.js"></script>
+<if expr="is_ios">
+    <!-- TODO(crbug.com/487000): Remove this once injected by web. -->
+    <script src="chrome://resources/js/ios/web_ui.js"></script>
+</if>
 <title>UKM Debug page</title>
 <h1>UKM Debug page</h1>
 <div>

components/ukm/debug/ukm_internals.js

@@ -75,4 +75,4 @@ function updateUkmData() {
   });
 }
 
-updateUkmData();
+document.addEventListener('DOMContentLoaded', updateUkmData);