This is useful to guarantee that the compiler can be run off-thread.

- Changes BaselineCompiler isolate_ private member type.
- Constructor still depends on Isolate, since it needs to construct
a MacroAssembler reference.

Bug: v8:12054
Change-Id: Ie7d18b8eb2d0b81a05713ce48d117f92796f2cde
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3085276
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76204}