Add missing USED_FROM_MULTIPLE_THREADS(EventSource) declaration.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@167818 bbb929c8-8fbe-4397-9dbb-9b2b20218538

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk