[NaCl SDK] Add URLLoader fake pepper interface.

BUG=244313,245431
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/77423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236343 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236343