[code coverage] Add code_coverage mixin to linux-chromeos-code-coverage

The code_coverage mixin is required to execute the code coverage
workflows.

NOTRY=true

Change-Id: I95d7b4dba5529d05688db98e3d03ce62f5957480
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524433
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641215}
2 files changed