Serializer enable/disable flags need thread safety.

src/arm/assembler-arm.cc

Index: src/arm/assembler-arm.cc
diff --git a/src/arm/assembler-arm.cc b/src/arm/assembler-arm.cc
index 516d3dd57ac3d7a4a282474b0c687ad576d66f53..a00fe7a43aa33e83fb9d2d3fa826aa1cdb14c38a 100644
--- a/src/arm/assembler-arm.cc
+++ b/src/arm/assembler-arm.cc
@@ -1079,11 +1079,6 @@ static bool fits_shifter(uint32_t imm32,
 // encoded.
 bool Operand::must_output_reloc_info(const Assembler* assembler) const {
   if (rmode_ == RelocInfo::EXTERNAL_REFERENCE) {
-#ifdef DEBUG
-    if (!Serializer::enabled()) {
-      Serializer::TooLateToEnableNow();
-    }
-#endif  // def DEBUG
     if (assembler != NULL && assembler->predictable_code_size()) return true;
     return Serializer::enabled();
   } else if (RelocInfo::IsNone(rmode_)) {
@@ -3267,11 +3262,6 @@ void Assembler::RecordRelocInfo(const RelocInfo& rinfo) {
   if (!RelocInfo::IsNone(rinfo.rmode())) {
     // Don't record external references unless the heap will be serialized.
     if (rinfo.rmode() == RelocInfo::EXTERNAL_REFERENCE) {
-#ifdef DEBUG
-      if (!Serializer::enabled()) {
-        Serializer::TooLateToEnableNow();
-      }
-#endif
       if (!Serializer::enabled() && !emit_debug_code()) {
         return;
       }