Add 2 more core OWNERS

Congratulations to Dan Sinclair and Xianzhu Wang,
use your new power wisely!

Also added them to rendering/OWNERS as this is what they
have been working on but grandfathered the OWNERS split.

Review URL: https://codereview.chromium.org/647753002

git-svn-id: svn://svn.chromium.org/blink/trunk@183549 bbb929c8-8fbe-4397-9dbb-9b2b20218538