IDL: Add IdlTypeBase.is_traceable

This is a follow-up CL of https://codereview.chromium.org/689013002

v8_union.py and v8_dictionary.py create a same flag from
the same condition. This should be a property of IdlType.

BUG=240176

Review URL: https://codereview.chromium.org/700603003

git-svn-id: svn://svn.chromium.org/blink/trunk@184814 bbb929c8-8fbe-4397-9dbb-9b2b20218538