IDL compiler: treat DOMString and DOMTimeStamp as primitive types

This is part of simplifying IncludesForType handling
(full CL was a bit long, breaking this off).
This removes various special cases for DOMString by treating it as a
primitive type.
Also DOMTimeStamp, which is just a typedef.

No changes to generated code.

BUG=239771

Review URL: https://codereview.chromium.org/31713002

git-svn-id: svn://svn.chromium.org/blink/trunk@160070 bbb929c8-8fbe-4397-9dbb-9b2b20218538