Fix support for mojo_common_unittests on MSan.
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338760}

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