IDL: All interface types are implicitly nullable

Previously, only the subset "wrapper types" were considered implicitly
nullable. The wrapper types are all interface types except some that all
are really also implicitly nullable.

This affects nullable attributes and method arguments of the types

  Dictionary
  EventListener
  EventHandler
  NodeFilter
  SerializedScriptValue

but since there are none currently, there are no code generation changes.

BUG=

Review URL: https://codereview.chromium.org/964163004

git-svn-id: svn://svn.chromium.org/blink/trunk@191065 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed