Mach port scopers should use get() instead of type conversion operators

Mach port scopers should use get() instead of type conversion operators

In https://codereview.chromium.org/1411523006, the Mach port scopers are
becoming better ScopedGenerics and are losing the type conversion
operators in the process. This is needed to adapt to that change. get()
is ugly, but being explicit about conversion isn’t a bad thing, and
these scopers will gain functionality such as Pass() as part of the
switch.

As a bonus, some would-be uses of get() to check for valid port rights
are becoming a more descriptive is_valid().

R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/6c0d42ce9dee55eaa906865191e28df35b32910d

[Mark Mentovai, 2015-10-16 05:49:10]

Patchset #2 (id:20001) has been deleted

[Mark Mentovai, 2015-10-16 05:49:30]

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

[Robert Sesek, 2015-10-20 15:01:54]

LGTM

[Mark Mentovai, 2015-10-20 15:03:30]

Committed patchset #3 (id:60001) manually as
6c0d42ce9dee55eaa906865191e28df35b32910d (presubmit successful).