Move attributes-related API from UniqueElementData to AttributeCollection

Source/core/core.gypi

 {
     'includes': [
       'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationPlayer.idl',
@@ skipping 1895 matching lines @@
             'xml/parser/XMLDocumentParserScope.h',
             'xml/parser/XMLErrors.cpp',
             'xml/parser/XMLErrors.h',
             'xml/parser/XMLParserInput.h',
         ],
         'webcore_dom_files': [
             'dom/ActiveDOMObject.cpp',
             'dom/AddConsoleMessageTask.cpp',
             'dom/AddConsoleMessageTask.h',
             'dom/Attr.cpp',
-            'dom/AttributeCollection.cpp',
             'dom/AttributeCollection.h',
             'dom/CDATASection.cpp',
             'dom/CDATASection.h',
             'dom/CSSSelectorWatch.cpp',
             'dom/CharacterData.cpp',
             'dom/ChildFrameDisconnector.cpp',
             'dom/ChildFrameDisconnector.h',
             'dom/ChildListMutationScope.cpp',
             'dom/ChildListMutationScope.h',
             'dom/ChildNodeList.cpp',
@@ skipping 1515 matching lines @@
             'rendering/RenderTableRowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }