Fix 'num cores' in ConcurrentMarking as well.

This is a follow-up to https://chromium-review.googlesource.com/904662
as I forgot this callsite there. The perf tests still haven't recovered
from decreasing the worker count by 1 to account for main thread
(crbug.com/809961) and I assume this line is at fault.

If this is correct, it also indicates ConcurrentMarking as a great
area to focus since a single extra worker appears to be making a
significant difference.

R=mlippautz@chromium.org

Bug: chromium:809961, chromium:808028
Change-Id: I9df933a4193fb25ea4e857f589e2164c8a2859b4
Reviewed-on: https://chromium-review.googlesource.com/911670Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51249}

src/heap/concurrent-marking.cc

@@ -520,8 +520,11 @@ void ConcurrentMarking::ScheduleTasks() {
   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() / 2);
+    task_count_ =
+        (static_cast<int>(
+             V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads()) +
+         1) /
+        2;
     task_count_ = Max(Min(task_count_, kMaxTasks), 1);
   }
   // Task id 0 is for the main thread.