Fix support for mojo_common_unittests on MSan.

This broke in 194f4c6bea6ae714cfa6a0eb32bf7e79a70e0192.

MSan uses libc++ on Linux, unlike the normal build, so libc++.so must be mapped.
It is mapped as part of base.isolate.

R=jam@chromium.org
BUG=98637

Review URL: https://codereview.chromium.org/1235673004

Cr-Commit-Position: refs/heads/master@{#338760}
1 file changed