Revert of Remove redundant activation threshold for incremental marking....

Revert of Remove redundant activation threshold for incremental marking. (patchset #1 id:1 of https://codereview.chromium.org/1418293006/ )

Reason for revert:
Regression in thread_times.key_idle_power_cases (crbug.com/555153)

Original issue's description:
> Remove redundant activation threshold for incremental marking.
>
> The minimum allocation limit already enforces this constraint for normal GCs.
>
> GCs triggered by the memory reducer and external limit should work for all heap sizes.
>
> BUG=chromium:552305
> LOG=NO
>
> Committed: https://crrev.com/e28e4d5f52852e2f14facc37a02069b6bfc82c4e
> Cr-Commit-Position: refs/heads/master@{#31921}

TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:552305

Review URL: https://codereview.chromium.org/1456523002

Cr-Commit-Position: refs/heads/master@{#32038}

src/heap/incremental-marking.cc

@@ -474,13 +474,21 @@ bool IncrementalMarking::WasActivated() { return was_activated_; }
 
 
 bool IncrementalMarking::CanBeActivated() {
+#ifndef DEBUG
+  static const intptr_t kActivationThreshold = 8 * MB;
+#else
+  // TODO(gc) consider setting this to some low level so that some
+  // debug tests run with incremental marking and some without.
+  static const intptr_t kActivationThreshold = 0;
+#endif
   // Only start incremental marking in a safe state: 1) when incremental
   // marking is turned on, 2) when we are currently not in a GC, and
   // 3) when we are currently not serializing or deserializing the heap.
   // Don't switch on for very small heaps.
   return FLAG_incremental_marking && heap_->gc_state() == Heap::NOT_IN_GC &&
          heap_->deserialization_complete() &&
-         !heap_->isolate()->serializer_enabled();
+         !heap_->isolate()->serializer_enabled() &&
+         heap_->PromotedSpaceSizeOfObjects() > kActivationThreshold;
 }