Introduce |IdlDictionary| class into |IdlDefinitions|.

This patch introduces |IdlDictionary| into |IdlDefinitions| to expose "dictionary" declarations to code generators.

R=nbarth
BUG=321462

Review URL: https://codereview.chromium.org/248753002

git-svn-id: svn://svn.chromium.org/blink/trunk@172355 bbb929c8-8fbe-4397-9dbb-9b2b20218538