Move CoverageInfo installation to the main-thread part of finalization.
Since this is debug-only, it won't affect non-debug compiles.

This allows us to use off-thread finalization when block coverage is
enabled, removing the last non-flag reason for disabling off-thread
finalization. This means we can remove the per-task "finalize
off-thread" flag on BackgroundCompileTask, and make off-thread
finalization always on when the flag is on.

Bug: chromium:1011762
Change-Id: Ib733d501c4043b493ded9a1951cceb5cfc524eb4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2497177
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70758}