Revert "Add field trial config for HighPriorityBeforeUnload."

This reverts commit 1b3c1be9.

Reason for revert: This is causing timeouts for lots of PDF tests on many dbg builders (see bug). I don't understand why but I have reproduced it locally and bisected and confirmed that this CL is culprit.

Original change's description:
> 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}

TBR=sky@chromium.org,kdillon@chromium.org

Change-Id: Id2c0f05304e515034efa769ded1bd38c61b1a53f
BUG: 1148674
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537275Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827190}

chrome/browser/lifetime/browser_close_manager_browsertest.cc

@@ -716,9 +716,8 @@ 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")));
-  // 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));
+  PrepareForDialog(browsers_[0]);
+  PrepareForDialog(browsers_[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.
 
-window.addEventListener('DOMContentLoaded', () => {
+onload = function() {
   var port = location.search.substr(1);
   var redirect =
       "http://127.0.0.1:" + port + "/server-redirect";
@@ -14,4 +14,4 @@ window.addEventListener('DOMContentLoaded', () => {
   link.download = "somefile.txt";
   document.body.appendChild(link);
   link.click();
-});
+};

content/public/test/browser_test_utils.cc

@@ -774,13 +774,8 @@ 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->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);
-    }
+    if (trigger_user_activation)
+      frame->ExecuteJavaScriptWithUserGestureForTests(base::string16());
 
     // 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

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