Updated WebTestWithWebState::LoadHtml comments.

Comments explain the reason for recursive LoadHtml retry.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Icd2b9a93e127500759c61a7e051e92ddc02df727
Reviewed-on: https://chromium-review.googlesource.com/926574
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537827}

ios/web/public/test/web_test_with_web_state.mm

@@ -125,7 +125,9 @@ void WebTestWithWebState::LoadHtml(NSString* html, const GURL& url) {
     return web_controller.loadPhase == PAGE_LOADED;
   });
 
-  // Reload the page if script execution is not possible.
+  // Reload the page if script execution is not possible. Script execution will
+  // fail if WKUserScript was not injected by WKWebView (which sometimes happens
+  // after -[WKWebView loadHTMLString:baseURL:]).
   if (![ExecuteJavaScript(@"0;") isEqual:@0]) {
     LoadHtml(html, url);
   }