The flag in question used to fall-back to Crankshaft whenever an OSR
request couldn't be handled by TurboFan. By now OSR in TurboFan is
sufficiently stabilized that one single --use-osr flag should do it.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/1960043002
Cr-Commit-Position: refs/heads/master@{#36102}