IDL: Use IdlNullableType wrapper to represent nullable types

Replaces the |is_nullable| flag previously stored in IdlTypeBase.

Review URL: https://codereview.chromium.org/474173002

git-svn-id: svn://svn.chromium.org/blink/trunk@180564 bbb929c8-8fbe-4397-9dbb-9b2b20218538