Move DocumentXMLTreeViewer off of private script

third_party/WebKit/Source/core/xml/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("xml") {
   sources = [
     "DOMParser.cpp",
     "DOMParser.h",
+    "DocumentXMLTreeViewer.cpp",
+    "DocumentXMLTreeViewer.h",
     "DocumentXPathEvaluator.cpp",
     "DocumentXPathEvaluator.h",
     "DocumentXSLT.cpp",
     "DocumentXSLT.h",
     "NativeXPathNSResolver.cpp",
     "NativeXPathNSResolver.h",
     "XMLSerializer.cpp",
     "XMLSerializer.h",
     "XPathEvaluator.cpp",
     "XPathEvaluator.h",
@@ skipping 43 matching lines @@
     "parser/XMLErrors.cpp",
     "parser/XMLErrors.h",
     "parser/XMLParserInput.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }