Add a assert for running a sanitizer in debug.
Add a assert for running a sanitizer in debug.

I found that msan doesn't link in debug builds. It fails linking with an
obscure error. To prevent other people from having this problem, this patch
adds an assert for this case.

Review URL: https://codereview.chromium.org/1326453003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#346764}

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