Do not force GC on each objects tracking sample.

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index c4da47cb48ea674d063eb7d71f23eee6614a6c2f..0c354c70eea024ced4008bc58cb21010de14c9d4 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -188,14 +188,12 @@ template <size_t ptr_size> struct SnapshotSizeConstants;
 template <> struct SnapshotSizeConstants<4> {
   static const int kExpectedHeapGraphEdgeSize = 12;
   static const int kExpectedHeapEntrySize = 24;
-  static const int kExpectedHeapSnapshotsCollectionSize = 100;
   static const int kExpectedHeapSnapshotSize = 132;
 };
 
 template <> struct SnapshotSizeConstants<8> {
   static const int kExpectedHeapGraphEdgeSize = 24;
   static const int kExpectedHeapEntrySize = 32;
-  static const int kExpectedHeapSnapshotsCollectionSize = 152;
   static const int kExpectedHeapSnapshotSize = 160;
 };
 
@@ -377,7 +375,8 @@ const SnapshotObjectId HeapObjectsMap::kFirstAvailableObjectId =
 HeapObjectsMap::HeapObjectsMap(Heap* heap)
     : next_id_(kFirstAvailableObjectId),
       entries_map_(AddressesMatch),
-      heap_(heap) {
+      heap_(heap),
+      last_gc_time_(0) {
   // This dummy element solves a problem with entries_map_.
   // When we do lookup in HashMap we see no difference between two cases:
   // it has an entry with NULL as the value or it has created
@@ -464,9 +463,17 @@ void HeapObjectsMap::StopHeapObjectsTracking() {
   time_intervals_.Clear();
 }
 
+
 void HeapObjectsMap::UpdateHeapObjectsMap() {
-  HEAP->CollectAllGarbage(Heap::kMakeHeapIterableMask,
-                          "HeapSnapshotsCollection::UpdateHeapObjectsMap");
+  double now = OS::TimeCurrentMillis();
+  if (now - last_gc_time_ > 1000) {
+    HEAP->CollectAllGarbage(Heap::kMakeHeapIterableMask,
+                            "HeapObjectsMap::UpdateHeapObjectsMap");
+    last_gc_time_ = now;
+  } else {
+    HEAP->EnsureHeapIsIterable();
+  }
+
   HeapIterator iterator(heap_);
   for (HeapObject* obj = iterator.next();
        obj != NULL;
@@ -655,9 +662,6 @@ Handle<HeapObject> HeapSnapshotsCollection::FindHeapObjectById(
 
 
 size_t HeapSnapshotsCollection::GetUsedMemorySize() const {
-  STATIC_CHECK(SnapshotSizeConstants<kPointerSize>::
-      kExpectedHeapSnapshotsCollectionSize ==
-      sizeof(HeapSnapshotsCollection));  // NOLINT
   size_t size = sizeof(*this);
   size += names_.GetUsedMemorySize();
   size += ids_.GetUsedMemorySize();