Reverted part and setPart.

part and setPart were implemented for ::part.
Since ::part was deprecated, part and setPart should be reverted.

BUG=321953

Review URL: https://codereview.chromium.org/85093002

git-svn-id: svn://svn.chromium.org/blink/trunk@162679 bbb929c8-8fbe-4397-9dbb-9b2b20218538