DevTools: NetworkRequest: clarify "initiator" type & nullability.

Previously inititor was accessed as a public member, so there were no
type info about it.

Now its type is specified.

BUG=

Review URL: https://codereview.chromium.org/529663002

git-svn-id: svn://svn.chromium.org/blink/trunk@181194 bbb929c8-8fbe-4397-9dbb-9b2b20218538