Add toDouble() helper, and use toFloat()/toDouble() for conversions

Source/bindings/core/v8/V8Binding.h

Index: Source/bindings/core/v8/V8Binding.h
diff --git a/Source/bindings/core/v8/V8Binding.h b/Source/bindings/core/v8/V8Binding.h
index 79ea8a9ce8f5e0c03dcab8a20c715d96e73aac26..000f5544abf557730e0a0a5f72140fbf3a50248d 100644
--- a/Source/bindings/core/v8/V8Binding.h
+++ b/Source/bindings/core/v8/V8Binding.h
@@ -535,6 +535,9 @@ inline float toFloat(v8::Local<v8::Value> value)
     return static_cast<float>(value->NumberValue());
 }
 
+// Convert a value to a double precision float, which might fail.
+double toDouble(v8::Handle<v8::Value>, ExceptionState&);
+
 // Converts a value to a String, throwing if any code unit is outside 0-255.
 String toByteString(v8::Handle<v8::Value>, ExceptionState&);