win: Add static_cast to AssignIfInRange

win: Add static_cast to AssignIfInRange

Otherwise, when assigning to a smaller type, MSVC warns e.g.

d:\src\crashpad\crashpad\util\numeric\safe_assignment.h(38) : error C2220: warning treated as error - no 'object' file generated
        d:\src\crashpad\crashpad\util\file\string_file_writer.cc(127) : see reference to function template instantiation 'bool crashpad::AssignIfInRange<size_t,FileOffset>(Destination *,Source)' being compiled
        with
        [
            Destination=size_t
,            Source=FileOffset
        ]
d:\src\crashpad\crashpad\util\numeric\safe_assignment.h(38) : warning C4244: '=' : conversion from 'FileOffset' to 'size_t', possible loss of data

R=mark@chromium.org
BUG=crashpad:1
Committed: https://chromium.googlesource.com/crashpad/crashpad/+/ec38bf152d53f29cdddd3384cddc161fc4fc963a

[Mark Mentovai, 2014-12-18 13:34:56]

LGTM

[scottmg, 2014-12-18 16:42:21]

Patchset #2 (id:20001) has been deleted

[scottmg, 2014-12-18 16:42:31]

Committed patchset #1 (id:1) manually as
ec38bf152d53f29cdddd3384cddc161fc4fc963a (presubmit successful).