Add counters for write barriers in optimized code.

src/mips/macro-assembler-mips.cc

Index: src/mips/macro-assembler-mips.cc
diff --git a/src/mips/macro-assembler-mips.cc b/src/mips/macro-assembler-mips.cc
index 397dc71b121935252f92beb9b9174339848fd521..a883281847b6660e8b1554a4c74139b55fa123ab 100644
--- a/src/mips/macro-assembler-mips.cc
+++ b/src/mips/macro-assembler-mips.cc
@@ -255,6 +255,12 @@ void MacroAssembler::RecordWrite(Register object,
         eq, kWrongAddressOrValuePassedToRecordWrite, at, Operand(value));
   }
 
+  // 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) {