This improvement to the const correctness makes ProgramBinary::save
compile with clang-cl as well, which is more strict than MSVC.

Change-Id: Iae091643fa1df7f094b89dbe7529c4e173f04a6d
Reviewed-on: https://chromium-review.googlesource.com/206714
Tested-by: <ehsan@mozilla.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>