This removes the fallback path in question. Now the {AstNumbering} phase
is the only phase deciding whether Crankshaft is supposed to be disabled
or not. This in turn simplifies reasoning about the paths through the
compilation pipeline. We can decide early whether we want Ignition to
kick in depending on whether Crankshaft is enabled or not.

R=mvstanton@chromium.org,rmcilroy@chromium.org

Review-Url: https://codereview.chromium.org/2146573004
Cr-Commit-Position: refs/heads/master@{#37763}