Make createHTMLDocument return a Document

As the FIXME says, instead of a HTMLDocument
we should return a plain Document:
https://dom.spec.whatwg.org/#domimplementation

Bug: 238368
Change-Id: I3dd9bf5039994ef0a98863dc0b4d4dffae23453d
Reviewed-on: https://chromium-review.googlesource.com/669459
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502867}

third_party/WebKit/Source/core/dom/DOMImplementation.cpp

@@ -201,7 +201,7 @@ bool DOMImplementation::IsTextMIMEType(const String& mime_type) {
          IsJSONMIMEType(mime_type) || IsTextPlainType(mime_type);
 }
 
-HTMLDocument* DOMImplementation::createHTMLDocument(const String& title) {
+Document* DOMImplementation::createHTMLDocument(const String& title) {
   DocumentInit init =
       DocumentInit::Create()
           .WithContextDocument(document_->ContextDocument())

third_party/WebKit/Source/core/dom/DOMImplementation.h

@@ -33,7 +33,6 @@ class Document;
 class DocumentInit;
 class DocumentType;
 class ExceptionState;
-class HTMLDocument;
 class XMLDocument;
 
 class CORE_EXPORT DOMImplementation final
@@ -58,7 +57,7 @@ class CORE_EXPORT DOMImplementation final
                               ExceptionState&);
 
   // From the HTMLDOMImplementation interface
-  HTMLDocument* createHTMLDocument(const String& title = String());
+  Document* createHTMLDocument(const String& title = String());
 
   // Other methods (not part of DOM)
   static Document* createDocument(const String& mime_type,

third_party/WebKit/Source/core/dom/DOMImplementation.idl

@@ -26,7 +26,7 @@
     [NewObject, RaisesException] DocumentType createDocumentType(DOMString qualifiedName, DOMString publicId, DOMString systemId);
     [NewObject, RaisesException] XMLDocument createDocument(DOMString? namespaceURI, [TreatNullAs=EmptyString] DOMString qualifiedName, optional DocumentType? doctype = null);
     // FIXME: createHTMLDocument should return a Document. crbug.com/238368
-    [NewObject] HTMLDocument createHTMLDocument(optional DOMString title);
+    [NewObject] Document createHTMLDocument(optional DOMString title);
 
     // useless; always returns true
     boolean hasFeature();