Remove FrameLoader::activeDocumentLoader()

It's a lookup for when you're too lazy to figure out whether you want the
provisional document loader or the committed document loader. Most use cases
very clearly want one loader or the other, but could end up with an irrelevant
provisional loader instead of a committed loader if a new navigation is in
progress.

BUG=none

Review URL: https://codereview.chromium.org/104023013

git-svn-id: svn://svn.chromium.org/blink/trunk@164466 bbb929c8-8fbe-4397-9dbb-9b2b20218538