Add finer-grained owners files

This patch adds finer-grained owners files for seven sub-directories of
Source/core and bootstraps them with the top owner contributors from the
past year [1]. A discussion of this can be found on blink-dev [2]. An
emeritus section has been added to core/OWNERS to signify less active
reviewers, though no entries have been removed.

[1] https://docs.google.com/a/chromium.org/spreadsheets/d/13DgGH9jbcHO5yRTOIYs6LBW5C1vDXYccrKfK6hbm7DE/edit#gid=0
[2] https://groups.google.com/a/chromium.org/d/msg/blink-dev/StkI3k4Pm5U/i5PdztzMgNMJ

Review URL: https://codereview.chromium.org/625053003

git-svn-id: svn://svn.chromium.org/blink/trunk@183511 bbb929c8-8fbe-4397-9dbb-9b2b20218538