Dampen the old generation allocation limit only after the initial old...

Dampen the old generation allocation limit only after the initial old generation size was configured.

Otherwise, dampening interferes with initial size configuration and causes GC too early.

BUG=501703
LOG=NO

Review URL: https://codereview.chromium.org/1183633005

Cr-Commit-Position: refs/heads/master@{#29106}

src/heap/heap.cc

@@ -1296,7 +1296,8 @@ bool Heap::PerformGarbageCollection(
     amount_of_external_allocated_memory_at_last_global_gc_ =
         amount_of_external_allocated_memory_;
     SetOldGenerationAllocationLimit(old_gen_size, gc_speed, mutator_speed);
-  } else if (HasLowYoungGenerationAllocationRate()) {
+  } else if (HasLowYoungGenerationAllocationRate() &&
+             old_generation_size_configured_) {
     DampenOldGenerationAllocationLimit(old_gen_size, gc_speed, mutator_speed);
   }