Use Document::isActive() instead of Document::renderer()

A bunch of random DOM code was poking at the render tree via
Document::renderer() when what it really wants to know is whether the document
is active. This CL removes some of these DOM -> RenderTree dependencies in
favor of Document::isActive(), which is tied into the Document's lifecycle.

R=esprehn
BUG=299011

Review URL: https://codereview.chromium.org/25504007

git-svn-id: svn://svn.chromium.org/blink/trunk@158675 bbb929c8-8fbe-4397-9dbb-9b2b20218538