IDL parser: add support for arbitrary length ExtAttr lists

Initially the Python IDL parser had a hard-coded limit of at most 2
Extended Attribute values -- A&B or A|B, but not A&B&C or A|B|C.
This was to keep the grammar simple, and sufficient for actual cases.
This CL removes the restriction, in response to a request by Alan.

Review URL: https://chromiumcodereview.appspot.com/22162002

git-svn-id: svn://svn.chromium.org/blink/trunk@155525 bbb929c8-8fbe-4397-9dbb-9b2b20218538