Remove possibility of stale user_buffer_ member in PPB_URLLoader_Impl when FinishedLoading() is called.

BUG=137778

Review URL: https://chromiumcodereview.appspot.com/10797037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@147914 0039d316-1c4b-4281-b951-d872f2087c98