DevTools: Label the more tabs button

When there are too many tabs to fit in the tab list, the button to see
more tabs was unlabeled.

Change-Id: Iadab77398a6bbf82d788d5a5e1cc0dc53ae1b589
Reviewed-on: https://chromium-review.googlesource.com/1188881Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586045}

third_party/blink/renderer/devtools/front_end/ui/TabbedPane.js

@@ -560,6 +560,8 @@ UI.TabbedPane = class extends UI.VBox {
   _createDropDownButton() {
     const dropDownContainer = createElementWithClass('div', 'tabbed-pane-header-tabs-drop-down-container');
     const chevronIcon = UI.Icon.create('largeicon-chevron', 'chevron-icon');
+    UI.ARIAUtils.markAsButton(dropDownContainer);
+    UI.ARIAUtils.setAccessibleName(dropDownContainer, ls`More tabs`);
     dropDownContainer.appendChild(chevronIcon);
     dropDownContainer.addEventListener('click', this._dropDownClicked.bind(this));
     dropDownContainer.addEventListener('mousedown', event => {