Add a few comments to core/OWNERS

We've now agreed to document areas of ownership in core/OWNERS
(https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/StkI3k4Pm5U/i5PdztzMgNMJ).
This adds a few descriptions of areas people gave me on a blink-core-owners-discuss thread
back in May.

NOTRY=True
BUG=None

Review URL: https://codereview.chromium.org/723063005

git-svn-id: svn://svn.chromium.org/blink/trunk@185379 bbb929c8-8fbe-4397-9dbb-9b2b20218538