IDL: Add more test cases to idl-dictionary-unittest.html

Add DictionaryHelper::getWithUndefinedOrNullCheck() to fix a bug that
newly added tests found. When IDL dictionary member is undefined or null,
the member should be treated as 'not present' and default values should
be used when specified. Since DictionaryHelper::get() converts
null/undefined into a valid value, we can't use DictionaryHelper::get()
in IDL dictionary implementation directly.

BUG=321462

Review URL: https://codereview.chromium.org/554893004

git-svn-id: svn://svn.chromium.org/blink/trunk@181604 bbb929c8-8fbe-4397-9dbb-9b2b20218538