IDL: Values for dictionaries should be a object, null or undefined

To follow the spec[1]. We should throw an type error when
a V8 value for IDL dictionary is neither an object, null, nor
undefined. The current implementation returns an
impl instance with default values.

[1] http://heycam.github.io/webidl/#es-dictionary

BUG=321462

Review URL: https://codereview.chromium.org/568703002

git-svn-id: svn://svn.chromium.org/blink/trunk@181891 bbb929c8-8fbe-4397-9dbb-9b2b20218538