win: Stub reorganization of MultiprocessExec

win: Stub reorganization of MultiprocessExec

This is moving towards having tests be in terms of MultiprocessExec
rather than Multiprocess/MachMultiprocess.

I had some difficulty in coming up with a way to do that directly,
so this patch is just some stubbing to move in hopefully the right
direction. In particular, makes multiprocess.h and
multiprocess_exec.h #include-able on Windows, makes the existing
.cc implementations of those POSIX-specific, and adds an
(unimplemented) stub of multiprocess_exec for Windows.

I think the subsequent steps are something like:
- properly implement for Windows
- make MachMultiprocess be implemented in terms of Exec (or possibly
  always have the "mach" bits available
- move all tests to use Exec
- merge Multiprocess and MultiprocessExec into one interface.

R=mark@chromium.org
BUG=crashpad:7

[scottmg, 2015-01-15 22:22:56]

scottmg@chromium.org changed reviewers:
+ mark@chromium.org

[scottmg, 2015-01-27 01:32:17]

Closing in favour of https://codereview.chromium.org/853853002/ which includes
the beginning of the Windows implementation.

[scottmg, 2015-01-27 01:32:36]

On 2015/01/27 01:32:17, scottmg wrote:
> Closing in favour of https://codereview.chromium.org/853853002/ which includes
> the beginning of the Windows implementation.

Er, in favour of https://codereview.chromium.org/880763002/ rather.