Add missing USED_FROM_MULTIPLE_THREADS(EventSource) declaration.

EventSource is available to Web Workers; declare its ThreadingTrait
accordingly.

R=
BUG=340522

Review URL: https://codereview.chromium.org/178913003

git-svn-id: svn://svn.chromium.org/blink/trunk@167818 bbb929c8-8fbe-4397-9dbb-9b2b20218538