Make new space allocation throughput estimation more accurate.

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index f0e2cf631489e0240691f8580fb863b46272fcc9..22c0316bce3f8a0b79100f39c274b92b45a36706 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -1570,7 +1570,7 @@ void SemiSpace::SetUp(Address start, int initial_capacity, int target_capacity,
   address_mask_ = ~(maximum_capacity - 1);
   object_mask_ = address_mask_ | kHeapObjectTagMask;
   object_expected_ = reinterpret_cast<uintptr_t>(start) | kHeapObjectTag;
-  age_mark_ = start_;
+  age_mark_ = start_ + NewSpacePage::kObjectStartOffset;
 }