ResourceFetcher:  Add POST support and the ability to set headers

These are needed to use the new Link Doctor API.

BUG=64832
R=darin@chromium.org, ttuttle@chromium.org

Review URL: https://codereview.chromium.org/140823010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247445 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247445