IDL: ArrayBuffer and ArrayBufferView support for union types

(ArrayBuffer or ArrayBufferView) is a common union type usage. For example,
CSS font loading[1] and fetch API[2] use it. Implement and use them for
FontFace constructor.

Change for FontFace is covered by fast/css/fontface-arraybuffer.html

This CL also fixed two minor bugs:
- Collect union types from constructors (we didn't before)
- We need to use 'implemented_as' for forward class declarations, instead of
  'name'.

[1] http://dev.w3.org/csswg/css-font-loading/#fontface-interface
[2] https://fetch.spec.whatwg.org/#body-mixin

BUG=240176

Review URL: https://codereview.chromium.org/699713003

git-svn-id: svn://svn.chromium.org/blink/trunk@184824 bbb929c8-8fbe-4397-9dbb-9b2b20218538