Tab Strip WebUI: Check for more than 1 active tab

Bug: 989131
Change-Id: I6ff22ccbbfc876dc6f43a757b700fecd901b2325
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848481
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704958}

chrome/browser/resources/tab_strip/tab_list.js

@@ -246,11 +246,17 @@ class TabListElement extends CustomElement {
    * @private
    */
   onTabActivated_(tabId) {
-    const previouslyActiveTab = this.getActiveTab_();
-    if (previouslyActiveTab) {
+    // There may be more than 1 TabElement marked as active if other events
+    // have updated a Tab to have an active state. For example, if a
+    // tab is created with an already active state, there may be 2 active
+    // TabElements: the newly created tab and the previously active tab.
+    this.shadowRoot.querySelectorAll('tabstrip-tab[active]')
+        .forEach((previouslyActiveTab) => {
+          if (previouslyActiveTab.tab.id !== tabId) {
             previouslyActiveTab.tab = /** @type {!TabData} */ (
                 Object.assign({}, previouslyActiveTab.tab, {active: false}));
           }
+        });
 
     const newlyActiveTab = this.findTabElement_(tabId);
     if (newlyActiveTab) {