[heap] Bump up the number of scavenging tasks

Increasing the number of scavenging tasks in the previous CL
https://chromium-review.googlesource.com/c/v8/v8/+/1697245
has helped with GC performance but increased memory usage.

This CL ensures that there are at least two scavenging tasks if the
new space size is larger than one MB.

Bug: chromium:982663,chromium:985731
Change-Id: I6edff8b90beb2740a5b75a51cf25f48aa5442482
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1733074Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63060}

src/heap/scavenger.cc

@@ -371,7 +371,7 @@ void ScavengerCollector::MergeSurvivingNewLargeObjects(
 int ScavengerCollector::NumberOfScavengeTasks() {
   if (!FLAG_parallel_scavenge) return 1;
   const int num_scavenge_tasks =
-      static_cast<int>(heap_->new_space()->TotalCapacity()) / MB;
+      static_cast<int>(heap_->new_space()->TotalCapacity()) / MB + 1;
   static int num_cores = V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1;
   int tasks =
       Max(1, Min(Min(num_scavenge_tasks, kMaxScavengerTasks), num_cores));