Create top level directory for the cross-platform updater project.

Bug: 924571
Change-Id: Iab05d07ab124240ae8d7b773565b059d623ba1db
Reviewed-on: https://chromium-review.googlesource.com/c/1423562
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625272}

chrome/updater/OWNERS

+borisv@chromium.org
+ganesh@chromium.org
+sorin@chromium.org
+waffles@chromium.org

chrome/updater/README.md

+This is the code for the client updater that will soon be used by
+desktop Chrome, on macOS and Windows.