Fix sandbox ASan test to cope with forward slashes in __FILE__
Fix sandbox ASan test to cope with forward slashes in __FILE__

gn invokes the compiler with forward slash paths while gyp does not.
This test failed when I ported WinASan to gn.

This CL doesn't use base::FilePath::BaseName because then I'd need to
get a wide string __FILE__, which is tricky.

R=wfh@chromium.org
BUG=598761

Review URL: https://codereview.chromium.org/1837403002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383990}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk