DevTools: handle devtools renderer unresponsiveness during beforeunload event interception

This patch fixes the crash which happenes under the following conditions:
1. DevTools window is in undocked state
2. DevTools renderer is unresponsive
3. User attempts to close inspected page

BUG=322380

Review URL: https://codereview.chromium.org/84883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237611 0039d316-1c4b-4281-b951-d872f2087c98
  NativeAppModalDialog* GetDialog() {
    AppModalDialog* dialog = ui_test_utils::WaitForAppModalDialog();
    EXPECT_TRUE(dialog->IsJavaScriptModalDialog());
    JavaScriptAppModalDialog* js_dialog =
        static_cast<JavaScriptAppModalDialog*>(dialog);
    NativeAppModalDialog* native_dialog = js_dialog->native_dialog();
    EXPECT_TRUE(native_dialog);
    return native_dialog;
  }

  NativeAppModalDialog* GetDialog() {
    AppModalDialog* dialog = ui_test_utils::WaitForAppModalDialog();
    EXPECT_TRUE(dialog->IsJavaScriptModalDialog());
    JavaScriptAppModalDialog* js_dialog =
        static_cast<JavaScriptAppModalDialog*>(dialog);
    NativeAppModalDialog* native_dialog = js_dialog->native_dialog();
    EXPECT_TRUE(native_dialog);
    return native_dialog;
  }
