Move SecurityContext to Document and WorkerGlobalScope superclass.

This change moves SecurityContext from superclass of ExecutionContext
to one of Document and WorkerGlobalScope. Now security origin
and CSP is provided to ExecutionContext from its clients.

In coming change, ExecutionContext will move to DOMWindow superclass
from Document superclass so that DOMWindow and scripting state
is shared between imported documents.

BUG=305497
R=abarth, dglazkov

Review URL: https://codereview.chromium.org/34793002

git-svn-id: svn://svn.chromium.org/blink/trunk@160307 bbb929c8-8fbe-4397-9dbb-9b2b20218538