This CL makes UnoptimizedCompilationJob a simple proxy for
BackgroundCompilerTask. A follow-up CL will remove UnoptimizedCompilationJob
entirely and have CompilerDispatcher deal directly with BackgroundCompilerTasks

BUG=v8:8041, v8:8015

Change-Id: Ia53d05c015c4ca2ee32a4d1c5d0c65edb3caeda8
Reviewed-on: https://chromium-review.googlesource.com/1236257
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56212}