Add api call to determine whether a string can be externalized.

src/api.cc

Index: src/api.cc
===================================================================
--- src/api.cc	(revision 2663)
+++ src/api.cc	(working copy)
@@ -2986,7 +2986,7 @@
   if (IsDeadCheck("v8::String::MakeExternal()")) return false;
   if (this->IsExternal()) return false;  // Already an external string.
   ENTER_V8;
-  i::Handle <i::String> obj = Utils::OpenHandle(this);
+  i::Handle<i::String> obj = Utils::OpenHandle(this);
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     // Operation was successful and the string is not a symbol. In this case
@@ -3022,7 +3022,7 @@
   if (IsDeadCheck("v8::String::MakeExternal()")) return false;
   if (this->IsExternal()) return false;  // Already an external string.
   ENTER_V8;
-  i::Handle <i::String> obj = Utils::OpenHandle(this);
+  i::Handle<i::String> obj = Utils::OpenHandle(this);
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     // Operation was successful and the string is not a symbol. In this case
@@ -3037,6 +3037,14 @@
 }
 
 
+bool v8::String::CanMakeExternal() {
+  if (IsDeadCheck("v8::String::CanMakeExternal()")) return false;
+  i::Handle<i::String> obj = Utils::OpenHandle(this);
+  int size = obj->Size();  // Byte size of the original string.
+  return (size >= i::ExternalString::kSize) && !obj->IsExternalString();
+}
+
+
 Local<v8::Object> v8::Object::New() {
   EnsureInitialized("v8::Object::New()");
   LOG_API("Object::New");