Add counters for write barriers in optimized code.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index b301f29e31f804cdca1817216414d1b3f354fa5e..0b285e7ff2b3eb45a9fee0fdf5750402271c9a6b 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -413,6 +413,10 @@ void MacroAssembler::RecordWrite(Register object,
     bind(&ok);
   }
 
+  // Count number of write barriers in generated code.
+  isolate()->counters()->write_barriers_static()->Increment();
+  IncrementCounter(isolate()->counters()->write_barriers_dynamic(), 1);
+
   // First, check if a write barrier is even needed. The tests below
   // catch stores of smis and stores into the young generation.
   Label done;