Teach the IDL compiler about TreatNullAs=EmptyString

Source/bindings/tests/idls/TestObject.idl

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary formstrArg, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 36 matching lines @@
     attribute TestEnumType             enumAttr;
     readonly attribute TestEnumType    readOnlyEnumAttr;
     attribute byte                     byteAttr;
     attribute octet                    octetAttr;
     attribute short                    shortAttr;
     attribute unsigned short           unsignedShortAttr;
     attribute long                     longAttr;
     attribute long long                longLongAttr;
     attribute unsigned long long       unsignedLongLongAttr;
     attribute DOMString                stringAttr;
+    [TreatNullAs=EmptyString] attribute DOMString treatNullAsEmptyStringStringAttr;
     [TreatNullAs=NullString] attribute DOMString treatNullAsNullStringStringAttr;
     [TreatNullAs=NullString, TreatUndefinedAs=NullString] attribute DOMString treatNullAsNullStringTreatUndefinedAsNullStringStringAttr;
     attribute EventHandler             eventHandlerAttr;
     [MeasureAs=TestFeature] attribute TestObject testObjAttr;
     [ExposeJSAccessors] attribute DOMString attrWithJSGetterAndSetter;
 
     // WK_ucfirst, WK_lcfirst exceptional cases.
     attribute TestObject                  XMLObjAttr;
     attribute boolean                  create;
 
     // Reflected DOM attributes
     [Reflect] attribute DOMString reflectedStringAttr;
+    [Reflect, TreatNullAs=EmptyString] attribute DOMString reflectedTreatNullAsEmptyStringStringAttr;
     [Reflect, TreatNullAs=NullString] attribute DOMString reflectedTreatNullAsNullStringStringAttr;
     [Reflect, TreatNullAs=NullString, TreatUndefinedAs=NullString] attribute DOMString reflectedTreatNullAsNullStringTreatUndefinedAsNullStringStringAttr;
     [Reflect] attribute long reflectedIntegralAttr;
     [Reflect] attribute unsigned long reflectedUnsignedIntegralAttr;
     [Reflect] attribute boolean reflectedBooleanAttr;
     [Reflect, URL] attribute DOMString reflectedURLAttr;
+    [Reflect, TreatNullAs=EmptyString, URL] attribute DOMString reflectedTreatNullAsEmptyStringURLAttr;
     [Reflect, TreatNullAs=NullString, URL] attribute DOMString reflectedTreatNullAsNullStringURLAttr;
     [Reflect, TreatNullAs=NullString, TreatUndefinedAs=NullString, URL] attribute DOMString reflectedTreatNullAsNullStringTreatUndefinedAsNullStringURLAttr;
     [Reflect=customContentStringAttr] attribute DOMString reflectedCustomStringAttr;
+    [Reflect=customContentStringAttr, TreatNullAs=EmptyString] attribute DOMString reflectedTreatNullAsEmptyStringCustomStringAttr;
     [Reflect=customContentStringAttr, TreatNullAs=NullString] attribute DOMString reflectedTreatNullAsNullStringCustomStringAttr;
     [Reflect=customContentStringAttr, TreatNullAs=NullString, TreatUndefinedAs=NullString] attribute DOMString reflectedTreatNullAsNullStringTreatUndefinedAsNullStringCustomStringAttr;
     [Reflect=customContentIntegralAttr] attribute long reflectedCustomIntegralAttr;
     [Reflect=customContentBooleanAttr] attribute boolean reflectedCustomBooleanAttr;
     [Reflect=customContentURLAttr, URL] attribute DOMString reflectedCustomURLAttr;
+    [Reflect=customContentURLAttr, TreatNullAs=EmptyString, URL] attribute DOMString reflectedTreatNullAsEmptyStringCustomURLAttr;
     [Reflect=customContentURLAttr, TreatNullAs=NullString, URL] attribute DOMString reflectedTreatNullAsNullStringCustomURLAttr;
     [Reflect=customContentURLAttr, TreatNullAs=NullString, TreatUndefinedAs=NullString, URL] attribute DOMString reflectedTreatNullAsNullStringTreatUndefinedAsNullStringCustomURLAttr;
     [Reflect, ReflectOnly="unique"] attribute DOMString limitedToOnlyOneAttribute;
     [Reflect, ReflectOnly="Per"|"Paal"|"Espen"] attribute DOMString limitedToOnlyAttribute;
     [Reflect=Other, ReflectOnly="Value1"|"Value2" ] attribute DOMString limitedToOnlyOtherAttribute;
     [Reflect, ReflectOnly="rsa" | "dsa", ReflectMissing="rsa"] attribute DOMString limitedWithMissingDefaultAttribute;
     [Reflect, ReflectOnly="ltr" | "rtl" | "auto", ReflectMissing="auto", ReflectInvalid="ltr"] attribute DOMString limitedWithInvalidMissingDefaultAttribute;
     [Reflect, ReflectOnly="left" | "right", ReflectMissing="left", ReflectInvalid="left"] attribute DOMString limitedWithInvalidAndMissingDefaultAttribute;
     [Reflect, ReflectOnly="anonymous"|"use-credentials", ReflectEmpty="anonymous", ReflectInvalid="anonymous"] readonly attribute DOMString corsSettingAttribute;
     [Reflect, ReflectOnly="empty"|"missing"|"invalid"|"a-normal", ReflectEmpty="empty", ReflectMissing="missing", ReflectInvalid="invalid"] readonly attribute DOMString limitedWithEmptyMissingInvalidAttribute;
@@ skipping 218 matching lines @@
     [DeprecateAs=Constant] const unsigned short DEPRECATED_CONSTANT = 1;
 
     // PutForwards
     [PutForwards=href] readonly attribute TestNode location;
     [PutForwards=hrefThrows] readonly attribute TestNode locationWithException;
 };
 
 // The following comment should not generate any code
 // TestObject implements TestImplements;