IDL compiler: [TreatReturnedNullStringAs]

Just adds one special case to v8_conversion_type,
but requires passing extended attributes down the call stack.

Also refactoring to simplify include handling in types
(input/output variable, not multiple return values,
gets rid of lots of "return foo, includes").

Docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-TreatReturnedNullStringAs-m-a-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/27464003

git-svn-id: svn://svn.chromium.org/blink/trunk@159747 bbb929c8-8fbe-4397-9dbb-9b2b20218538