Refactoring: Introduce HTMLImport interface to form import tree.

Source/core/core.gypi

 {
     'variables': {
         '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 2070 matching lines @@
             'html/HTMLHeadElement.cpp',
             'html/HTMLHeadingElement.cpp',
             'html/HTMLHeadingElement.h',
             'html/HTMLHtmlElement.cpp',
             'html/HTMLHtmlElement.h',
             'html/HTMLIFrameElement.cpp',
             'html/HTMLImageElement.cpp',
             'html/HTMLImageElement.h',
             'html/HTMLImageLoader.cpp',
             'html/HTMLImageLoader.h',
+            'html/HTMLImport.cpp',
+            'html/HTMLImport.h',
             'html/HTMLImportsController.cpp',
             'html/HTMLImportsController.h',
             'html/HTMLInputElement.cpp',
             'html/HTMLKeygenElement.cpp',
             'html/HTMLKeygenElement.h',
             'html/HTMLLIElement.cpp',
             'html/HTMLLIElement.h',
             'html/HTMLLabelElement.cpp',
             'html/HTMLLabelElement.h',
             'html/HTMLLegendElement.cpp',
@@ skipping 1643 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++"'
               }],
          ],
     }
 }