Avoid incremental marking write-barrier when constructing descriptor arrays.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index d0d3354b800e3eb5f228f7bde7a941bddef6ad9f..c20becaf1d8c85dd31fca87f3e5b7e98f35d0afb 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -50,7 +50,8 @@ IncrementalMarking::IncrementalMarking(Heap* heap)
       steps_took_since_last_gc_(0),
       should_hurry_(false),
       allocation_marking_factor_(0),
-      allocated_(0) {
+      allocated_(0),
+      no_marking_scope_depth_(0) {
 }
 
 
@@ -702,6 +703,8 @@ void IncrementalMarking::Step(intptr_t allocated_bytes) {
 
   if (allocated_ < kAllocatedThreshold) return;
 
+  if (state_ == MARKING && no_marking_scope_depth_ > 0) return;
+
   intptr_t bytes_to_process = allocated_ * allocation_marking_factor_;
 
   double start = 0;