[Isolates] Convert more static data either to read-only or to per-isolate.

src/runtime.h

Index: src/runtime.h
diff --git a/src/runtime.h b/src/runtime.h
index a1e7c879ddf07a5cb2f7c3969f6e5c0c93316023..470033992ffb6520f6681279b0f1d0f25c6312c5 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -544,10 +544,10 @@ class Runtime : public AllStatic {
                                                   Object* dictionary);
 
   // Get the intrinsic function with the given name, which must be a symbol.
-  static Function* FunctionForSymbol(Handle<String> name);
+  static const Function* FunctionForSymbol(Handle<String> name);
 
   // Get the intrinsic function with the given FunctionId.
-  static Function* FunctionForId(FunctionId id);
+  static const Function* FunctionForId(FunctionId id);
 
   // General-purpose helper functions for runtime system.
   static int StringMatch(Isolate* isolate,