Simplify implementation of Mutex.

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index ae8a0b58ba87939a2adf6eb2298826082843cb27..ab957be9f9efa0a9d3af3454f5ab01d1f19dfd4c 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -891,7 +891,7 @@ void ExternalReference::SetUp() {
   double_constants.the_hole_nan = BitCast<double>(kHoleNanInt64);
   double_constants.negative_infinity = -V8_INFINITY;
 
-  math_exp_data_mutex = OS::CreateMutex();
+  math_exp_data_mutex = new Mutex;
 }
 
 
@@ -899,7 +899,7 @@ void ExternalReference::InitializeMathExpData() {
   // Early return?
   if (math_exp_data_initialized) return;
 
-  math_exp_data_mutex->Lock();
+  ScopedLock with(math_exp_data_mutex);
   if (!math_exp_data_initialized) {
     // If this is changed, generated code must be adapted too.
     const int kTableSizeBits = 11;
@@ -935,7 +935,6 @@ void ExternalReference::InitializeMathExpData() {
 
     math_exp_data_initialized = true;
   }
-  math_exp_data_mutex->Unlock();
 }