Add Caitlin Fischer as a Chromium metrics owner

I have been reverse shadowing her reviews for a few weeks, and I think
she is ready to become a full-fledged owner.

R=asvitkine

Bug: None
Change-Id: I01b629b3e1312a85df11b193a7196c230f3452d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2543831
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828447}

base/metrics/OWNERS

@@ -6,6 +6,7 @@
 
 asvitkine@chromium.org
 bcwhite@chromium.org
+caitlinfischer@google.com
 holte@chromium.org
 isherman@chromium.org
 jwd@chromium.org