Add OWNERS file for //chrome/browser/undo

Add original author of the code and original reviewer as OWNERS and
sdefresne@ for componentization work of //chrome/browser/undo.

BUG=476919
R=sky@chromium.org

Review URL: https://codereview.chromium.org/1088513005

Cr-Commit-Position: refs/heads/master@{#325453}

chrome/browser/undo/OWNERS

+sky@chromium.org
+tom.cassiotis@gmail.com
+
+# For refactoring only during componentization of //chrome/browser/undo.
+sdefresne@chromium.org