Add ByteString support to IDL bindings

Source/bindings/v8/V8Binding.cpp

Index: Source/bindings/v8/V8Binding.cpp
diff --git a/Source/bindings/v8/V8Binding.cpp b/Source/bindings/v8/V8Binding.cpp
index a03187eabab6280a96c799868359f8952fe9a002..61acaf5cd2c074300f0d579b442faf3125d9caf2 100644
--- a/Source/bindings/v8/V8Binding.cpp
+++ b/Source/bindings/v8/V8Binding.cpp
@@ -492,6 +492,16 @@ float toFloat(v8::Handle<v8::Value> value, ExceptionState& exceptionState)
     return numberObject->NumberValue();
 }
 
+String toByteString(v8::Handle<v8::Value> value, ExceptionState& exceptionState)
+{
+    String string = toCoreString(value.As<v8::String>());

[jsbell, 2014/05/30 21:53:28]

Yeah, this As<v8::String> is bogus if it's not a string. I'll dig further.

[Nils Barth (inactive), 2014/06/02 03:20:16]

Should it be ToString() instead then?
(JS values are all stringifable.)
Anyway, that's what the spec says, right?:
http://heycam.github.io/webidl/#es-ByteString

In any case, could you link to the spec here (per above URL),
and quote line-by-line so the spec <-> code correspondence is explicit?
(V. useful if the spec changes!)

E.g. (with whatever the correct code is):
// 1. Let x be ToString(V).
String string = value.ToString();
// 2. If the value of any element of x is greater than 255, then throw a
TypeError.
if (!...)

[jsbell, 2014/06/03 23:22:36]

Yeah; handling should be identical to js->DOMString, except for the extra
throwing case.

That does mean the ToString needs to handle the conversion itself throwing. Was
afraid of the macros, but I've got that sorted now.

+    if (!string.containsOnlyLatin1()) {
+        exceptionState.throwTypeError("Value is not a valid ByteString.");
+        return String();
+    }
+    return string;
+}
+
 PassRefPtrWillBeRawPtr<XPathNSResolver> toXPathNSResolver(v8::Handle<v8::Value> value, v8::Isolate* isolate)
 {
     RefPtrWillBeRawPtr<XPathNSResolver> resolver = nullptr;