decode_test_driver: Use size_t for size arg to DecodeFrame().

Use size_t for DecodeFrame()'s size arg, and cast only
at the vpx_codec_decode() call site. This silences warnings that
appear in svc_test.cc when building with vs2013.

Change-Id: I2cf39f02a45732c752097f07b0c7ad414b1517d8
2 files changed