Also removes can_execute_on_background_thread() since all compilation jobs can now do that.
Part of the work towards enabling off-thread bytecode compilation.

BUG=v8:5203

Change-Id: I6a52c26d599ce74482b5fb49926603cb326f1e31
Reviewed-on: https://chromium-review.googlesource.com/731285Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48826}