The Windows implementation of CaptureContext used a macro to refer to
the offset of a field in a struct.
Bug: chromium:762167
Change-Id: I621d5c88283b1d066158559aade8811a9825c72e
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2426743
Reviewed-by: Mark Mentovai <mark@chromium.org>