Revert "Revert "Use all available workers for concurrent marking.""

This reverts commit c41c7a09.

Reason for revert: relanding now that the perf waterfall has had a stab at this revert.

Original change's description:
> Revert "Use all available workers for concurrent marking."
> 
> This reverts commit 3c62f7ae.
> (and commit 4939463c)
> 
> The goal of this revert is to contrast the effect on perf bots of
> landing it vs reverting it to more easily attribute its impact.
> 
> R=​hpayer@chromium.org
> 
> Bug: chromium:812178
> Change-Id: I7c977b1b0b587f787263272400d87f6aae7af634
> Reviewed-on: https://chromium-review.googlesource.com/936761
> Commit-Queue: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51546}

TBR=gab@chromium.org,hpayer@chromium.org

Change-Id: I1ecfc70867dc5424cba1a9ecd229ae031c3e9aa4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:812178
Reviewed-on: https://chromium-review.googlesource.com/937725Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51577}

src/heap/concurrent-marking.cc

@@ -506,14 +506,10 @@ void ConcurrentMarking::ScheduleTasks() {
   base::LockGuard<base::Mutex> guard(&pending_lock_);
   DCHECK_EQ(0, pending_task_count_);
   if (task_count_ == 0) {
-    // TODO(ulan): Increase the number of tasks for platforms that benefit
-    // from it.
-    task_count_ =
-        (static_cast<int>(
-             V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads()) +
-         1) /
-        2;
-    task_count_ = Max(Min(task_count_, kMaxTasks), 1);
+    task_count_ = Max(
+        1, Min(kMaxTasks,
+               static_cast<int>(V8::GetCurrentPlatform()
+                                    ->NumberOfAvailableBackgroundThreads())));
   }
   // Task id 0 is for the main thread.
   for (int i = 1; i <= task_count_; i++) {

src/heap/concurrent-marking.h

@@ -53,7 +53,10 @@ class ConcurrentMarking {
     COMPLETE_TASKS_FOR_TESTING,
   };
 
-  static constexpr int kMaxTasks = 4;
+  // TODO(gab): The only thing that prevents this being above 7 is
+  // Worklist::kMaxNumTasks being maxed at 8 (concurrent marking doesn't use
+  // task 0, reserved for the main thread).
+  static constexpr int kMaxTasks = 7;
   using MarkingWorklist = Worklist<HeapObject*, 64 /* segment size */>;
 
   ConcurrentMarking(Heap* heap, MarkingWorklist* shared,