Teach InspectorInstrumentation how to turn a Node into a Document

This makes the InspectorDOMAgent callbacks less verbose and avoids taking
the address of Node::document() in lots of places.

Also added an inspectorAgentsFor(Document&) callback which avoids
NULL-checking the document.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/27066002

git-svn-id: svn://svn.chromium.org/blink/trunk@159786 bbb929c8-8fbe-4397-9dbb-9b2b20218538