[NaCl SDK] nacl_io: Don't use PPB_FileRef_1_2

At least not until we need too, since it requires
a recent chrome version.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3779

Review URL: https://codereview.chromium.org/160073003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250755 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250755