Fix a race condition in simulator that happens when flushing icache.

During GC multiple threads can request icache flush when evacuating
code space in parallel.

Simulator::FlushICache updates Isolate::simulator_icache hashmap,
which leads to a race.

This patch adds a lock for simulator_icache.

BUG=

Review-Url: https://codereview.chromium.org/2338793002
Cr-Commit-Position: refs/heads/master@{#39386}

src/assembler.cc

@@ -190,6 +190,7 @@ void AssemblerBase::FlushICache(Isolate* isolate, void* start, size_t size) {
   if (size == 0) return;
 
 #if defined(USE_SIMULATOR)
+  base::LockGuard<base::Mutex> lock_guard(isolate->simulator_i_cache_mutex());
   Simulator::FlushICache(isolate->simulator_i_cache(), start, size);
 #else
   CpuFeatures::FlushICache(start, size);

src/isolate.h

@@ -1164,6 +1164,10 @@ class Isolate {
 
   PRINTF_FORMAT(2, 3) void PrintWithTimestamp(const char* format, ...);
 
+#ifdef USE_SIMULATOR
+  base::Mutex* simulator_i_cache_mutex() { return &simulator_i_cache_mutex_; }
+#endif
+
  protected:
   explicit Isolate(bool enable_serializer);
   bool IsArrayOrObjectPrototype(Object* object);
@@ -1440,6 +1444,10 @@ class Isolate {
   v8::Isolate::AbortOnUncaughtExceptionCallback
       abort_on_uncaught_exception_callback_;
 
+#ifdef USE_SIMULATOR
+  base::Mutex simulator_i_cache_mutex_;
+#endif
+
   friend class ExecutionAccess;
   friend class HandleScopeImplementer;
   friend class OptimizingCompileDispatcher;