Pepper: Enable some FileRef,URLLoader tests on Mac

Some tests were disabled on Mac 10.7 during the FileRef refactor. These issues
were fixed, not buggy test behavior, and the tests should be re-enabled.

BUG=280570

Review URL: https://chromiumcodereview.appspot.com/23519014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224723 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224723