- dbghelp.h requires windows.h to be included before it (ick!).
Add a stub one for non_win to make this work.
- convert __attribute__ -> macro that can work work with MSVC;
- a handful of narrowing casts.
R=mark@chromium.org
BUG=crashpad:1
Review URL: https://codereview.chromium.org/883773005
