Add ConstThreadState to mach_extensions.h and use it everywhere

Add ConstThreadState to mach_extensions.h and use it everywhere.

TEST=everything
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/40b1d7cb1d9e22bdc588dd8d2fe76e5688e41616

[Mark Mentovai, 2015-04-01 16:34:57]

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

[Mark Mentovai, 2015-04-01 16:34:57]

Following up from https://codereview.chromium.org/1049023003/, I noticed that we
already had a ConstThreadState in client/simulate_crash_mac.cc. It’s a net win
of 8 lines to move it into util and use it everywhere.

[Mark Mentovai, 2015-04-01 16:38:52]

The 8 lines was actually just clang-format being a jerk. This is really a gain
of 10 lines. 💩

[Robert Sesek, 2015-04-02 19:26:39]

LGTM.

I didn't check IWYU everywhere, though.

[Mark Mentovai, 2015-04-02 19:28:32]

Committed patchset #2 (id:20001) manually as
40b1d7cb1d9e22bdc588dd8d2fe76e5688e41616 (presubmit successful).