WebUI Tab Strip: Fix possible cause of flaky tests

There may be a race condition for when the JavaScript to query a child
in the shadow DOM of a tabstrip-tab-group element runs immediately after
the test adds a tab to a group. This CL waits for the tabstrip-tab-group
element to appear on the page before running this query.

Bug: 1132300
Change-Id: I228f694ffeffa5c9bf303f4952fbbabef8f2334e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594507
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837839}

chrome/browser/ui/webui/tab_strip/tab_strip_ui_browsertest.cc

@@ -110,15 +110,29 @@ IN_PROC_BROWSER_TEST_F(TabStripUIBrowserTest, ActivatingTabClosesEmbedder) {
                               ISOLATED_WORLD_ID_CHROME_INTERNAL));
 }
 
-// Flaky.
-// TODO(https://crbug.com/1132300): Re-enable.
-IN_PROC_BROWSER_TEST_F(TabStripUIBrowserTest,
-                       DISABLED_InvokesEditDialogForGroups) {
+IN_PROC_BROWSER_TEST_F(TabStripUIBrowserTest, InvokesEditDialogForGroups) {
   using ::testing::_;
 
   tab_groups::TabGroupId group_id =
       browser()->tab_strip_model()->AddToNewGroup({0});
 
+  // Wait for the front-end to receive the new group and create the tab-group
+  // element.
+  const std::string get_group_promise_js =
+      "new Promise((resolve) => {"
+      "  const interval = setInterval(() => {"
+      "    if (document.querySelector('tabstrip-tab-list').shadowRoot"
+      "        .querySelector('tabstrip-tab-group')) {"
+      "      resolve(true);"
+      "      clearInterval(interval);"
+      "    }"
+      "  }, 100);"
+      "});";
+  ASSERT_TRUE(content::EvalJs(webui_contents_.get(), get_group_promise_js,
+                              content::EXECUTE_SCRIPT_DEFAULT_OPTIONS,
+                              ISOLATED_WORLD_ID_CHROME_INTERNAL)
+                  .ExtractBool());
+
   const std::string get_chip_js =
       "const chip = document.querySelector('tabstrip-tab-list')"
       "    .shadowRoot.querySelector('tabstrip-tab-group')"