Split CustomElementRegistry into a registration context and a registry.

Source/core/core.gypi

 {
     'variables': {
         'deprecated_perl_core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSMediaRule.idl',
@@ skipping 1729 matching lines @@
             'dom/CustomElementCallbackDispatcher.h',
             'dom/CustomElementCallbackInvocation.cpp',
             'dom/CustomElementCallbackInvocation.h',
             'dom/CustomElementCallbackQueue.cpp',
             'dom/CustomElementCallbackQueue.h',
             'dom/CustomElementDefinition.cpp',
             'dom/CustomElementDefinition.h',
             'dom/CustomElementDescriptor.h',
             'dom/CustomElementDescriptorHash.h',
             'dom/CustomElementLifecycleCallbacks.h',
+            'dom/CustomElementRegistrationContext.cpp',
+            'dom/CustomElementRegistrationContext.h',
             'dom/CustomElementRegistry.cpp',
             'dom/CustomElementRegistry.h',
             'dom/CustomElementUpgradeCandidateMap.cpp',
             'dom/CustomElementUpgradeCandidateMap.h',
             'dom/CustomEvent.cpp',
             'dom/CustomEvent.h',
             'dom/DOMError.cpp',
             'dom/DOMError.h',
             'dom/DOMException.cpp',
             'dom/DOMException.h',
@@ skipping 2009 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }