LocalHeap can be used on main thread, however allocation might cause a
GC which works differently on the main thread than on a background
thread. Support collection on main thread by directly performing the GC
instead of requesting the GC as done on background threads.

To allow for differentiation between main and background threads,
LocalHeap/LocalIsolate now require an additional argument.

Change-Id: I08094ea633e303e149913f21dff395da9e046534
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2463238Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70590}