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

src/code-stubs.h

Index: src/code-stubs.h
diff --git a/src/code-stubs.h b/src/code-stubs.h
index a1e015812549de6234352a85b10e6e133a144691..f17fa07202d3e08f9aa1133c58732a21d3e527e4 100644
--- a/src/code-stubs.h
+++ b/src/code-stubs.h
@@ -54,6 +54,7 @@ namespace internal {
   V(StubFailureTrampoline)                  \
   V(SubString)                              \
   V(ToNumber)                               \
+  V(ToString)                               \
   V(ToObject)                               \
   V(VectorStoreICTrampoline)                \
   V(VectorKeyedStoreICTrampoline)           \
@@ -3082,6 +3083,15 @@ class ToNumberStub final : public PlatformCodeStub {
 };
 
 
+class ToStringStub final : public PlatformCodeStub {
+ public:
+  explicit ToStringStub(Isolate* isolate) : PlatformCodeStub(isolate) {}
+
+  DEFINE_CALL_INTERFACE_DESCRIPTOR(ToString);
+  DEFINE_PLATFORM_CODE_STUB(ToString, PlatformCodeStub);
+};
+
+
 class ToObjectStub final : public HydrogenCodeStub {
  public:
   explicit ToObjectStub(Isolate* isolate) : HydrogenCodeStub(isolate) {}