Cleanup: remove temporary define for multi-sided patch in WebServiceWorkerContextClient.h

Once https://codereview.chromium.org/273423003/ lands
we no longer need HAS_SERVICE_WORKER_CONTEXT_DESTROYED define

BUG=366617
TEST=no functional changes
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/281533002

git-svn-id: svn://svn.chromium.org/blink/trunk@173934 bbb929c8-8fbe-4397-9dbb-9b2b20218538