Remove [TreatNullAs=NullString] from data attribute on CharacterData

Replace NullString with EmptyString of data atrribute as per spec[1].
[1]: https://dom.spec.whatwg.org/#dom-characterdata-data

Remove the null check in setData() because binding-generated code
has the null check on data.

Bug: 497307
Change-Id: I35ff0e0f864c2bc300f551bef6f25c1908c8a2ac
Reviewed-on: https://chromium-review.googlesource.com/1172308
Commit-Queue: Amos Lim <eui-sang.lim@samsung.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583011}

third_party/blink/renderer/core/dom/character_data.cc

@@ -41,11 +41,9 @@ void CharacterData::Atomize() {
 }
 
 void CharacterData::setData(const String& data) {
-  const String& non_null_data = !data.IsNull() ? data : g_empty_string;
   unsigned old_length = length();
 
-  SetDataAndUpdate(non_null_data, 0, old_length, non_null_data.length(),
-                   kUpdateFromNonParser);
+  SetDataAndUpdate(data, 0, old_length, data.length(), kUpdateFromNonParser);
   GetDocument().DidRemoveText(*this, 0, old_length);
 }
 
@@ -171,7 +169,7 @@ bool CharacterData::ContainsOnlyWhitespace() const {
 }
 
 void CharacterData::setNodeValue(const String& node_value) {
-  setData(node_value);
+  setData(!node_value.IsNull() ? node_value : g_empty_string);
 }
 
 void CharacterData::SetDataAndUpdate(const String& new_data,

third_party/blink/renderer/core/dom/character_data.idl

@@ -20,7 +20,7 @@
 // https://dom.spec.whatwg.org/#interface-characterdata
 
 interface CharacterData : Node {
-    attribute [TreatNullAs=NullString] DOMString data;
+    attribute [TreatNullAs=EmptyString] DOMString data;
     readonly attribute unsigned long length;
     [RaisesException] DOMString substringData(unsigned long offset, unsigned long count);
     void appendData(DOMString data);