Remove DebugName from the public API

Now that cc is getting debug name info for layers via debug info, we can remove
the related plumbing in blink. This depends on
https://codereview.chromium.org/135493002/

R=jamesr@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/135503002

git-svn-id: svn://svn.chromium.org/blink/trunk@165227 bbb929c8-8fbe-4397-9dbb-9b2b20218538