vp8_decrypt_test.c: Silence MSVC data loss warning.

- Change type of encrypt_buffer() offset argument to ptrdiff_t, and change the
  type of the size argument to size_t.
- Update size argument encrypt_buffer() in vp8_boolcoder_test.c with
  same.

Change-Id: Ie29c7c82c73318bee01b89c6fb4c4e1442eef03c