Don't expose DocumentWriter to embedding layer

Currently, several classes that need to build up a fake frame do so by getting direct access to a DocumentWriter and sending the document contents directly into the parsing system. This works, but it looks like a terrible layering violation, particularly since it involves reusing whatever DocumentLoader happens to be the committed DocumentLoader (generally the DocumentLoader for the initial empty document).

This patch re-routes these cases to use SubstituteData and go through the normal FrameLoader::load() codepath. This involves re-introducing the concept of a SubstituteData load that is forced to be synchronous, but that seems like less of a hack than the current system.

BUG=

Review URL: https://codereview.chromium.org/120513004

git-svn-id: svn://svn.chromium.org/blink/trunk@164624 bbb929c8-8fbe-4397-9dbb-9b2b20218538