Serializer enable/disable flags need thread safety.

src/arm64/assembler-arm64.cc

Index: src/arm64/assembler-arm64.cc
diff --git a/src/arm64/assembler-arm64.cc b/src/arm64/assembler-arm64.cc
index b399277543ff039911c1884e7c022d288fb401e7..0e5241281fe216a9eb8d9c8b92a4b025cb7982c4 100644
--- a/src/arm64/assembler-arm64.cc
+++ b/src/arm64/assembler-arm64.cc
@@ -274,9 +274,7 @@ void Operand::initialize_handle(Handle<Object> handle) {
 bool Operand::NeedsRelocation() const {
   if (rmode_ == RelocInfo::EXTERNAL_REFERENCE) {
 #ifdef DEBUG
-    if (!Serializer::enabled()) {
-      Serializer::TooLateToEnableNow();
-    }
+    Serializer::TrySetPermanentlyDisabled();
 #endif
     return Serializer::enabled();
   }
@@ -1971,7 +1969,7 @@ void Assembler::debug(const char* message, uint32_t code, Instr params) {
   // might be run on real hardware.
   if (!Serializer::enabled()) {
 #ifdef DEBUG
-    Serializer::TooLateToEnableNow();
+    Serializer::TrySetPermanentlyDisabled();
 #endif
     // The arguments to the debug marker need to be contiguous in memory, so
     // make sure we don't try to emit pools.
@@ -2526,9 +2524,7 @@ void Assembler::RecordRelocInfo(RelocInfo::Mode rmode, intptr_t data) {
     // Don't record external references unless the heap will be serialized.
     if (rmode == RelocInfo::EXTERNAL_REFERENCE) {
 #ifdef DEBUG
-      if (!Serializer::enabled()) {
-        Serializer::TooLateToEnableNow();
-      }
+      Serializer::TrySetPermanentlyDisabled();
 #endif
       if (!Serializer::enabled() && !emit_debug_code()) {
         return;