[heap] Introduce enum of garbage collection reasons.

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 46dc4f428426ea1c2f093cf08b04955a1b0fa7d6..e95f87d23d02585452d472c6eb09e3b082b0a921 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -2558,8 +2558,8 @@ HeapObject* FreeList::Allocate(int size_in_bytes) {
   // if it is big enough.
   owner_->EmptyAllocationInfo();
 
-  owner_->heap()->StartIncrementalMarkingIfNeeded(
-      Heap::kNoGCFlags, kNoGCCallbackFlags, "old space step");
+  owner_->heap()->StartIncrementalMarkingIfAllocationLimitIsReached(
+      Heap::kNoGCFlags, kNoGCCallbackFlags);
 
   int new_node_size = 0;
   FreeSpace* new_node = FindNodeFor(size_in_bytes, &new_node_size);
@@ -2995,8 +2995,8 @@ AllocationResult LargeObjectSpace::AllocateRaw(int object_size,
     reinterpret_cast<Object**>(object->address())[1] = Smi::FromInt(0);
   }
 
-  heap()->StartIncrementalMarkingIfNeeded(Heap::kNoGCFlags, kNoGCCallbackFlags,
-                                          "old space step");
+  heap()->StartIncrementalMarkingIfAllocationLimitIsReached(Heap::kNoGCFlags,
+                                                            kNoGCCallbackFlags);
   AllocationStep(object->address(), object_size);
 
   if (heap()->incremental_marking()->black_allocation()) {