Make clang the default on linux.
Make clang the default on linux.

TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/781983002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26164}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh