Refactored the final parts of the TurboFan pipeline a bit.

Refactored the final parts of the TurboFan pipeline a bit.

This removed some inconsistent/duplicated code and made the testing
side entrance much simpler: Now it simply constructs a pipeline and
hands off to the "tail" of the normal pipeline.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/924823002

Cr-Commit-Position: refs/heads/master@{#26637}