Improve 'XMLHttpRequest' exception messages.

BUG=270033

Review URL: https://chromiumcodereview.appspot.com/23041002

git-svn-id: svn://svn.chromium.org/blink/trunk@156212 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce XMLHttpRequestEventTarget IDL interface

Introduce XMLHttpRequestEventTarget IDL interface to match the latest specification:
- http://xhr.spec.whatwg.org/#xmlhttprequesteventtarget

It avoids duplication of the event handlers between XMLHttpRequest and
XMLHttpRequestUpload.

BUG=273698

Review URL: https://chromiumcodereview.appspot.com/23243002

git-svn-id: svn://svn.chromium.org/blink/trunk@156213 bbb929c8-8fbe-4397-9dbb-9b2b20218538