DevTools: Roll Closure compiler to 8b4b812b275ea3bf5cf296a2df94dede03609120

Most prominent changes:
  - Multiple externs improvements
  - Promises support
  - Improved type inference
  - ES6 support (to a certain degree)
  - Custom coding conventions enforcement
  - Improved parser API (used by JSDocValidator)

This patch also incorporates:
  - Updated JSDocValidator (due to the new Closure parser API);
  - JS modifications (due to the updated Closure externs).

R=aandrey@chromium.org, sergeyv@chromium.org, vsevik@chromium.org, aandrey, sergeyv, vsevik

Review URL: https://codereview.chromium.org/339353002

git-svn-id: svn://svn.chromium.org/blink/trunk@176597 bbb929c8-8fbe-4397-9dbb-9b2b20218538