This prepares the code-base so that Ignition can be enabled on a certain
subset of compilations without setting the {FLAG_ignition} flag (which
enables Ignition on all compilations). We should not check the flag in
question explicitly anywhere outside of the compiler heuristics.

R=mvstanton@chromium.org

Review-Url: https://codereview.chromium.org/2443573002
Cr-Commit-Position: refs/heads/master@{#40617}