rAc: make requestAutocomplete() return a promise.

Source/core/core.gypi

 {
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationPlayer.idl',
             'animation/TimedItem.idl',
             'animation/Timeline.idl',
             'animation/Timing.idl',
             'clipboard/DataTransfer.idl',
@@ skipping 28 matching lines @@
             'css/RGBColor.idl',
             'css/Rect.idl',
             'css/StyleMedia.idl',
             'css/StyleSheet.idl',
             'css/StyleSheetList.idl',
             'css/WebKitCSSFilterRule.idl',
             'css/WebKitCSSFilterValue.idl',
             'css/WebKitCSSMatrix.idl',
             'css/WebKitCSSTransformValue.idl',
             'dom/Attr.idl',
+            'dom/AutocompleteError.idl',
             'dom/CDATASection.idl',
             'dom/CharacterData.idl',
             'dom/ClientRect.idl',
             'dom/ClientRectList.idl',
             'dom/Comment.idl',
             'dom/DOMError.idl',
             'dom/DOMException.idl',
             'dom/DOMImplementation.idl',
             'dom/DOMSettableTokenList.idl',
             'dom/DOMStringList.idl',
@@ skipping 1852 matching lines @@
             'xml/parser/XMLDocumentParser.h',
             'xml/parser/XMLDocumentParserScope.cpp',
             'xml/parser/XMLDocumentParserScope.h',
             'xml/parser/XMLParserInput.h',
         ],
         'webcore_dom_files': [
             'dom/ActiveDOMObject.cpp',
             'dom/AddConsoleMessageTask.cpp',
             'dom/AddConsoleMessageTask.h',
             'dom/Attr.cpp',
+            'dom/AutocompleteError.cpp',
+            'dom/AutocompleteError.h',
             'dom/CDATASection.cpp',
             'dom/CDATASection.h',
             'dom/CSSSelectorWatch.cpp',
             'dom/CharacterData.cpp',
             'dom/ChildListMutationScope.cpp',
             'dom/ChildListMutationScope.h',
             'dom/ChildNodeList.cpp',
             'dom/ChildNodeList.h',
             'dom/ClassCollection.cpp',
             'dom/ClassCollection.h',
@@ skipping 1458 matching lines @@
             'html/track/vtt/VTTScannerTest.cpp',
             'loader/MixedContentCheckerTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }