[Cronet] Add mgersh@ to Cronet OWNERS file

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

R=mef@chromium.org

Review-Url: https://codereview.chromium.org/2587573002
Cr-Commit-Position: refs/heads/master@{#439501}

components/cronet/OWNERS

 kapishnikov@chromium.org
 mef@chromium.org
 mmenke@chromium.org
+mgersh@chromium.org
 pauljensen@chromium.org
 xunjieli@chromium.org