Report more details in heap snapshot progress indicator

When loading heap snapshot (either from file or backend) report separate progress for loading nodes, edges, strings and each processing step.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/23717022

git-svn-id: svn://svn.chromium.org/blink/trunk@157080 bbb929c8-8fbe-4397-9dbb-9b2b20218538
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