REGRESSION(r157081): Fix multipart handling of ResourceFetcher

r157081 broke multipart handling in ResourceFetcher.
The breakage was harmless except Motion JPEG case - ResourceFetcher
lost the reference to Motion JPEG resource so it was no longer
able to cancel the ongoing resource fetching.

This change makes the tracking working again.
Also, the change adds a stopFetching() call to DocumentLoder::stopLoading()
that is used to be there but was lost at some point.

TEST=stop-loading.html
R=japhet@chromium.org,abarth@chromium.org
BUG=309641

Review URL: https://codereview.chromium.org/242373002

git-svn-id: svn://svn.chromium.org/blink/trunk@171969 bbb929c8-8fbe-4397-9dbb-9b2b20218538