ShaderVariable's functions and members are all mixed up. This CL
reorders things to move the functions before the members, to better
match ANGLE's coding style.

This also adds a note that any new members that are added need to be
added to gl::WriteShaderVar() and gl::LoadShaderVar(), which are
actually implemented in Program.cpp, since ShaderVars.h/cpp can't
include libANGLE/BinaryStream.h. There are currently several members
that are missing, leading to save/load errors, which are fixed in a
follow-on CL in this chain.

Bug: b/182409935
Change-Id: I1381a5efffb56c0989961bbcfd405b95d59d6396
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2770682
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>