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We only use prebuilt instrumented libraries with msan, so use the right
default for that, and asser that it's not used in any other build configs.

Bug: 1002971
Change-Id: I23c8099394af711a9e28027f9c3093f9f6fd18b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1797468
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Evgenii Stepanov <eugenis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695889}