Simplify the logic in the CompilerDispatcher to use BackgroundCompileTasks
directly, rather than having a (now unecessary) CompilerDispatcherJob
abstraction. In the process, the CompilerDispatcherTracer is removed, and the
idle task logic is simplified finalize already compiled jobs until the
idle task deadline.

BUG=v8:8238, v8:8041

Change-Id: I1ea2366f959b6951de222d62fde80725b3cc70ff
Reviewed-on: https://chromium-review.googlesource.com/c/1260123
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56473}