net: Use SequencedTaskRunner for URLFetcherResponseWriter

TBR=scottbyer@chromium.org for chrome/service/cloud_print/cloud_print_url_fetcher.h
BUG=316065
TEST=build

Review URL: https://codereview.chromium.org/63923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234357 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234357