Building and running tests with v8_enabled_concurrent_marking=false
currently produces two failures:
1) Segmentation fault on attempt to mark a read-only object.
   This is fixed by changing MarkBit::Set to be a no-op if the object
   is already marked (which is the case for the readonly space).
2) Missing write-barrier due to bogus condition in the bailout.
   The barrier can be skipped only if the host object is not marked yet.

This also disables two concurrent allocation tests that rely on
concurrent marking write-barrier.

Bug: v8:10875

Change-Id: Ib3a238fc34c8f20c697470e0bd4ac427fb4bdc0e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421816Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70041}