Now that UnoptimizedCompileJob only has three stages, move the logic for
stepping between these stages out of UnoptimizedCompileJob and back into
CompilerDispatcher.

BUG=v8:5203

Change-Id: I3bb776e14ef9da801dc9792e9e643b8026135060
Reviewed-on: https://chromium-review.googlesource.com/774743Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49422}