Remove support for custom pseudo elements.

This is a revert of https://src.chromium.org/viewvc/blink?view=rev&revision=168627.

BUG=254020

Review URL: https://codereview.chromium.org/215173003

git-svn-id: svn://svn.chromium.org/blink/trunk@170210 bbb929c8-8fbe-4397-9dbb-9b2b20218538