IDL compiler: More argument type exceptions (optional Dictionary, Promise)

This adds support for 2 more special cases:
* optional Dictionary
* Promise
...and fixes some typos in the Dictionary argument template. (>.<)

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/50523014

git-svn-id: svn://svn.chromium.org/blink/trunk@161031 bbb929c8-8fbe-4397-9dbb-9b2b20218538