Change SecurityOrigin from ThreadSafeRefCounted to RefCounted

SecurityOrigin should be thread-local and therefore RefCounted.

BUG=415865

Review URL: https://codereview.chromium.org/665893002

git-svn-id: svn://svn.chromium.org/blink/trunk@185270 bbb929c8-8fbe-4397-9dbb-9b2b20218538