Add WPT tests for inert

Bug: 692360
Change-Id: Ie549ed3d33bd7f2cb7c5be070f931e29a2cb106b
Reviewed-on: https://chromium-review.googlesource.com/1139846
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581425}
9 files changed