IDL compiler: treat DOMTimeStamp as a typedef

DOMTimeStamp is a typedef, not a type.
Thus we should handle it in the parser (typedefs are translated during
parse, and are invisible to the CG).
This removes some special cases from the CG, and makes the parser more
spec-compilant.
http://www.w3.org/TR/WebIDL/#common-DOMTimeStamp

BUG=239771

Review URL: https://codereview.chromium.org/25438004

git-svn-id: svn://svn.chromium.org/blink/trunk@158605 bbb929c8-8fbe-4397-9dbb-9b2b20218538