Add WebKit::WebDOMError interface in Blink API

Currently DOMError is not exposed at Blink API so it cannot be
passed to/from or constructed in chromium.  This CL adds WebDOMError
interface to Blink API so that client code can handle DOMError
object.

BUG=313131

Review URL: https://codereview.chromium.org/50363004

git-svn-id: svn://svn.chromium.org/blink/trunk@160947 bbb929c8-8fbe-4397-9dbb-9b2b20218538