Add counters for write barriers in optimized code.

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
diff --git a/src/arm/macro-assembler-arm.cc b/src/arm/macro-assembler-arm.cc
index 25060ea823b20fe7a40ba84f0b0ab165df7926a7..a1524347195f803ce2102df7bc8888b3f82b4241 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -484,6 +484,12 @@ void MacroAssembler::RecordWrite(Register object,
     Check(eq, kWrongAddressOrValuePassedToRecordWrite);
   }
 
+  // Count number of write barriers in generated code.
+  isolate()->counters()->write_barriers_static()->Increment();
+  // TODO(mstarzinger): Dynamic counter missing.
+
+  // First, check if a write barrier is even needed. The tests below
+  // catch stores of smis and stores into the young generation.
   Label done;
 
   if (smi_check == INLINE_SMI_CHECK) {