Move ResourceLoaderSet from DocumentLoader to ResourceFetcher.

This is a part of the fetch refactoring series.
This change:
- promotes ResourceLoaderSet from a typedef to a real class.
- moves DocumentLoader::m_resourceLaoders and m_multipartResourceLoaders
  to ResourceFetcher, which creates Resouce objects and naturally knows
  their loaders' lifetime.

This reduces the number of DocumentLoader dependency from ResourceFetcher.

BUG=280939
R=japhet,abarth
TEST=none

Review URL: https://chromiumcodereview.appspot.com/23444011

git-svn-id: svn://svn.chromium.org/blink/trunk@157081 bbb929c8-8fbe-4397-9dbb-9b2b20218538