Add field trial config for HighPriorityBeforeUnload.

Change-Id: Ia274d2e51503514c3fc98420fa4eee5eb040e1fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490782
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826894}

chrome/browser/lifetime/browser_close_manager_browsertest.cc

@@ -716,8 +716,9 @@ IN_PROC_BROWSER_TEST_F(BrowserCloseManagerBrowserTest,
   AddBlankTabAndShow(browsers_[1]);
   ASSERT_NO_FATAL_FAILURE(ui_test_utils::NavigateToURL(
       browsers_[1], embedded_test_server()->GetURL("/beforeunload.html")));
-  PrepareForDialog(browsers_[0]);
-  PrepareForDialog(browsers_[1]);
+  // Prep the new tabs for dialog to ensure the beforeunload dialog will show.
+  PrepareForDialog(browsers_[0]->tab_strip_model()->GetWebContentsAt(1));
+  PrepareForDialog(browsers_[1]->tab_strip_model()->GetWebContentsAt(1));
   ASSERT_NO_FATAL_FAILURE(AcceptClose());
   ASSERT_NO_FATAL_FAILURE(CancelClose());
   cancel_observer.Wait();

chrome/test/data/extensions/api_test/webnavigation/download/a.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-onload = function() {
+window.addEventListener('DOMContentLoaded', () => {
   var port = location.search.substr(1);
   var redirect =
       "http://127.0.0.1:" + port + "/server-redirect";
@@ -14,4 +14,4 @@ onload = function() {
   link.download = "somefile.txt";
   document.body.appendChild(link);
   link.click();
-};
+});

content/public/test/browser_test_utils.cc

@@ -743,8 +743,13 @@ bool WaitForLoadStop(WebContents* web_contents) {
 void PrepContentsForBeforeUnloadTest(WebContents* web_contents,
                                      bool trigger_user_activation) {
   for (auto* frame : web_contents->GetAllFrames()) {
-    if (trigger_user_activation)
-      frame->ExecuteJavaScriptWithUserGestureForTests(base::string16());
+    if (trigger_user_activation && frame->IsRenderFrameLive()) {
+      // Execute script with user interaction to prep the page to show
+      // before unload dialog if needed. Must wait on a response from the
+      // renderer before proceeding to ensure the page has been interacted with.
+
+      ExecuteScriptWithUserGestureControl(frame, std::string(), true);
+    }
 
     // Disable the hang monitor, otherwise there will be a race between the
     // beforeunload dialog and the beforeunload hang timer.

testing/variations/fieldtrial_testing_config.json

@@ -1098,6 +1098,26 @@
             ]
         }
     ],
+    "BlinkSchedulerHighPriorityBeforeUnload": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "HighPriorityBeforeUnload"
+                    ]
+                }
+            ]
+        }
+    ],
     "BlinkSchedulerPerAgentSchedulingStudy": [
         {
             "platforms": [