Clean up FrameLoader/DocumentLoader commit logic

This is primarily moving stuff that is currently in FrameLoader::clear() and DocumentLoader::createWriterFor(), reducing the number of paramters on these functions and moving parts of them to more conceptually accurate locations.

BUG=none

Review URL: https://codereview.chromium.org/106893008

git-svn-id: svn://svn.chromium.org/blink/trunk@164616 bbb929c8-8fbe-4397-9dbb-9b2b20218538