Add Gayane to the list of metrics reviewers.

Note: Since the last time you were in OWNERS for histograms.xml,
we've refactored our OWNERS files and have a central file in
base/metrics. So this gives you OWNERS over a lot more files than
just histograms.xml - please defer to other OWNERS if reviews
come in for code you're not familiar with. And please review
the docs we have at:

https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/histograms/README.md

For best practices for histograms that you should hold up during
review. Thanks!

BUG=none

Change-Id: I05441f908958b3312b54c4b1420834f57c8209ef
Reviewed-on: https://chromium-review.googlesource.com/776281Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517438}

base/metrics/OWNERS

 asvitkine@chromium.org
+gayane@chromium.org
 holte@chromium.org
 isherman@chromium.org
 jwd@chromium.org