Add leviw as a platform OWNER

Levi is already an OWNER and owns a lot of the code that has now been
moved to platform.

R=eseidel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/139043002

git-svn-id: svn://svn.chromium.org/blink/trunk@165104 bbb929c8-8fbe-4397-9dbb-9b2b20218538