webxr-tests: change media/ to ../media/

This is in preparation for serving versioned test pages for
cross-version testing. For local serving, the extra '../' will
be ignored by the server.

Bug: 923149
Change-Id: Ieb23b397d331de6babc41c94132d1b924cec956d
Reviewed-on: https://chromium-review.googlesource.com/c/1455305
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630013}
21 files changed