This is the third of three layered patches that introduce a new content management API to the email package.  This patch is layered on top of the patch in issue 18860.  Unfortunately the fact that these patches are layered (which I did in hopes of making them into more reviewable chunks) means that only the first one gets a rietveld link.

This patch provides the raw_data_manager, and makes it the default content_manager.  The raw_data_manager doesn't provide any facilities beyond that provided by the base API of email package, but it does provide those facilities in a much more convenient and consistent API.