Fix the URLBlacklistManager on iOS.

URLRequests never have net::LOAD_MAIN_FRAME nor net::LOAD_SUB_FRAME set on that
platform, so the blacklist must filter all URLRequests that it sees for now.

BUG=338283
R=dconnelly@chromium.org

Review URL: https://codereview.chromium.org/148253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247240 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247240