Rather than storing the isolate in compiler dispatcher jobs, which gets
weird for jobs that are entirely off-thread, instead pass the isolate
in when stepping on the main thread.

This makes it clearer which steps must be executed on the main thread,
as they require the caller to explicitly give them access to the
isolate.

Bug: v8:6537
Change-Id: I02fff7c77fdcdbfb099a38235f94d8c1040699ac
Reviewed-on: https://chromium-review.googlesource.com/589437
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46962}