Reland: Add a trace event when pausing/preempting concurrent marking.

Reland reason : not the culprit.

This will let us identify in traces whether unwinding after receiving
the preemption event is slower than desired and should be optimized.

Adding it to pausing while working on removing it in
https://chromium-review.googlesource.com/c/v8/v8/+/922103
will allow gathering traces that highlight the issue.

R=ulan@chromium.org

Bug: chromium:812178
Change-Id: I0dc0f6754980157674968ba4a868f12c779e69bc
Reviewed-on: https://chromium-review.googlesource.com/923989Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51372}

src/heap/concurrent-marking.cc

@@ -479,8 +479,12 @@ void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
         base::AsAtomicWord::Relaxed_Store<size_t>(&task_state->marked_bytes,
                                                   marked_bytes);
         if (task_state->interrupt_request.Value()) {
+          TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                       "ConcurrentMarking::Run Paused");
           task_state->interrupt_condition.Wait(&task_state->lock);
         } else if (task_state->preemption_request.Value()) {
+          TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                       "ConcurrentMarking::Run Preempted");
           break;
         }
       }