[Desktop Android] Use TabListInterface::CloseTab() in browsertests

Use TabListInterface::CloseTab() to close tabs in ExtensionBrowserTest.
This reduces the need for bespoke per-platform code.

While we're at it, also properly wait for the WebContents to close,
reducing potential test flakiness.

Bug: 427503497
Change-Id: I431f0615542bb8cddac0d9e138a48b746c972356
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6854742
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525811}