Add yzshen@ as a mojo/ owner.

I would like to do this because:
- the mojo/ folder is now more focused after services were moved out of the
folder;
- make it a little easier whenever people want me to review public/ stuff but at
the same time need to change GYP files, isolated files, etc.
- I will still mostly focus on reviewing public/ and bindings-related code; but
happy to review other things if needed.

BUG=None

Review-Url: https://codereview.chromium.org/1937113002
Cr-Commit-Position: refs/heads/master@{#391084}

mojo/OWNERS

@@ -2,3 +2,4 @@ amistry@chromium.org
 ben@chromium.org
 rockot@chromium.org
 sky@chromium.org
+yzshen@chromium.org