Basic support for unrestricted types

This allows one to use 'unrestricted float' and 'unrestricted double' in
IDL files: they are distinguished by the frontend (they have different
types), but for now the CG generates the same output.

I'll follow this up with [TypeChecking=Unrestricted], which will do type
checking for float and double, but not unrestricted float or unrestricted
double.

R=haraken
BUG=354298

Review URL: https://codereview.chromium.org/257783003

git-svn-id: svn://svn.chromium.org/blink/trunk@172603 bbb929c8-8fbe-4397-9dbb-9b2b20218538