IDL compiler: add enumerations

For attribute getters, enumerations are just a matter of extracting them
from the parsed IDL file, and then treating them as DOMStrings
appropriately.
For setters we'll need to add a check statement (only set valid values),
but that's later.

Also a bit of rearranging in types, to clearly distinguish
general IDL type issues from V8-specific ones.

BUG=239771

Review URL: https://codereview.chromium.org/25671002

git-svn-id: svn://svn.chromium.org/blink/trunk@158704 bbb929c8-8fbe-4397-9dbb-9b2b20218538