Reland "Add browsertests for tab sharing UI views."

This is a reland of 916c54c5

Original change's description:
> Add browsertests for tab sharing UI views.
>
> Add tests to verify tab sharing UI elements: infobars, contents border
> and tab sharing indicator.
>
> Bug: 996722
> Change-Id: I5fa5ec7e4a48a66c1ba9b08ff76b84a105961352
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847287
> Reviewed-by: Guido Urdaneta <guidou@chromium.org>
> Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#707368}

Bug: 996722
Change-Id: Ic325cb95314cbe299253895f15534a83ef8b193e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1878409Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709017}

chrome/test/BUILD.gn

@@ -1948,6 +1948,7 @@ if (!is_android) {
         "../browser/ui/views/status_bubble_views_browsertest_mac.mm",
         "../browser/ui/views/sync/inline_login_ui_browsertest.cc",
         "../browser/ui/views/sync/profile_signin_confirmation_dialog_views_browsertest.cc",
+        "../browser/ui/views/tab_sharing/tab_sharing_ui_views_browsertest.cc",
         "../browser/ui/views/tabs/tab_group_editor_bubble_view_browsertest.cc",
         "../browser/ui/views/task_manager_view_browsertest.cc",
         "../browser/ui/views/toolbar/browser_actions_container_browsertest.cc",