[Structured metrics] Extend presubmit XML validation to cover projects.

This adds presubmit validation coverage to the new projects section of
the structured.xml file. As well as checking owners etc., it also
ensures that all events reference either no project or a valid project.
This is also implicitly checked by the fact that chrome won't compile
otherwise, but two checks don't hurt :)

Bug: 1034842
Change-Id: Idd73483c37a5976cf1173eb029d45cc8d46a0aa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2114770
Commit-Queue: Tony Yeoman <tby@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753032}