[runtime] Add %ToString and %_ToString and remove the TO_STRING builtin.

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index 2e77930bea307aed01d51b4ffe664c29351a8fdd..1168fe9d94ab99b6adc2ddadf46fd1f0f2b39875 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -25,6 +25,7 @@ class PlatformInterfaceDescriptor;
   V(FastNewClosure)                           \
   V(FastNewContext)                           \
   V(ToNumber)                                 \
+  V(ToString)                                 \
   V(ToObject)                                 \
   V(NumberToString)                           \
   V(Typeof)                                   \
@@ -363,6 +364,16 @@ class ToNumberDescriptor : public CallInterfaceDescriptor {
 };
 
 
+class ToStringDescriptor : public CallInterfaceDescriptor {
+ public:
+  enum ParameterIndices { kReceiverIndex };
+
+  DECLARE_DESCRIPTOR(ToStringDescriptor, CallInterfaceDescriptor)
+
+  static const Register ReceiverRegister();
+};
+
+
 class ToObjectDescriptor : public CallInterfaceDescriptor {
  public:
   enum ParameterIndices { kReceiverIndex };