win: Fix Clang -Wsign-compare warnings in new test code
win: Fix Clang -Wsign-compare warnings in new test code

This test code appeared in 9609b7471676, and was missed by the similar
warning cleanup of a51e912004a6, which was developed in parallel.

Bug: crashpad:192, chromium:779790
Change-Id: I4ed88ed025e4be4410c98ceaca395218f00007be
Reviewed-on: https://chromium-review.googlesource.com/750024
Reviewed-by: Robert Sesek <f55ad94e336f3a28322d0a25d280221ea9daf7a3@chromium.org>

atom/crashpad,chromium/crashpad,chromium/crashpad,chromium/crashpad,atom/crashpad,atom/crashpad