[Parser] Introduce AstStringConstants to share constants across AstValueFactory

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index fc36de3aa53b0a64f3d1cf51d8636ce14704eacc..6e3b9f5bc7bf57c5da6a7b7594de5b3c1ce8d458 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -35,6 +35,7 @@ namespace internal {
 
 class AccessCompilerData;
 class AddressToIndexHashMap;
+class AstStringConstants;
 class BasicBlockProfiler;
 class Bootstrapper;
 class CancelableTaskManager;
@@ -1157,6 +1158,10 @@ class Isolate {
     return cancelable_task_manager_;
   }
 
+  AstStringConstants* ast_string_constants() const {
+    return ast_string_constants_;
+  }
+
   interpreter::Interpreter* interpreter() const { return interpreter_; }
 
   AccountingAllocator* allocator() { return allocator_; }
@@ -1399,6 +1404,8 @@ class Isolate {
   std::unique_ptr<CodeEventDispatcher> code_event_dispatcher_;
   FunctionEntryHook function_entry_hook_;
 
+  AstStringConstants* ast_string_constants_;
+
   interpreter::Interpreter* interpreter_;
 
   CompilerDispatcher* compiler_dispatcher_;