Fix a 64-bit error in ProgramBinary::load

ProgramBinary::save writes a size_t, so ProgramBinary::load needs
to read it as the same type.

Landed for jschuh.
Original review: https://codereview.appspot.com/7304057/
Review URL: https://codereview.appspot.com/7310052

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1825 736b8ea6-26fd-11df-bfd4-992fa37f6226