This prepares for https://codereview.chromium.org/2188693002/

With the current configuration, sanitizers wouldn't be used
at all. The clang configuration is automatically derived
in GN.

BUG=chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2184683004
Cr-Commit-Position: refs/heads/master@{#38089}