Add a LocalIsolate for the main thread to Isolate. This LocalIsolate is
kept alive during the whole lifetime of the Isolate. The main thread
LocalIsolate starts in the Running state in contrast to the background
thread LocalIsolates (those start in Parked).

Code paths in Turbofan that used to create a LocalIsolate on the main
thread can now simply use the main thread LocalIsolate.

LocalIsolate for the main thread will help in reducing differences
between the main and background threads. The goal is that the main
thread behaves more like a background thread.

The main thread LocalIsolate should also make it simpler to share code
between main thread and background threads by using LocalIsolate for
both.

Bug: v8:10315
Change-Id: I7fd61d305a6fd7079e2319d75c291c1021e70018
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2509593Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71226}