This removes the ability of the compilation pipeline to invoke the
Crankshaft optimizing compiler for JavaScript functions. Note that in
this state Crankshaft can still be used to compile code stubs.

R=rmcilroy@chromium.org
BUG=v8:6408

Change-Id: I0bec7c8ec7c705c13257df43796403a228ea631c
Reviewed-on: https://chromium-review.googlesource.com/527443Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45803}