Exposes Symbol.toPrimitive in C++ APIs as Symbol::GetToPrimitive.

As Blink needs to set Symbol.toPrimitive, exposes the symbol in C++ APIs
as Symbol::GetToPrimitive.

BUG=chromium:680409

Review-Url: https://codereview.chromium.org/2657933003
Cr-Commit-Position: refs/heads/master@{#42724}

include/v8.h

@@ -2710,6 +2710,7 @@ class V8_EXPORT Symbol : public Name {
   // Well-known symbols
   static Local<Symbol> GetIterator(Isolate* isolate);
   static Local<Symbol> GetUnscopables(Isolate* isolate);
+  static Local<Symbol> GetToPrimitive(Isolate* isolate);
   static Local<Symbol> GetToStringTag(Isolate* isolate);
   static Local<Symbol> GetIsConcatSpreadable(Isolate* isolate);
 

src/api.cc

@@ -7834,6 +7834,10 @@ Local<Symbol> v8::Symbol::GetUnscopables(Isolate* isolate) {
   return Utils::ToLocal(i_isolate->factory()->unscopables_symbol());
 }
 
+Local<Symbol> v8::Symbol::GetToPrimitive(Isolate* isolate) {
+  i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
+  return Utils::ToLocal(i_isolate->factory()->to_primitive_symbol());
+}
 
 Local<Symbol> v8::Symbol::GetToStringTag(Isolate* isolate) {
   i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);