Add support for BeforeUnloadEvent

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/CSSKeyframeRule.idl',
@@ skipping 23 matching lines @@
             'css/StyleSheetList.idl',
             'css/WebKitCSSFilterRule.idl',
             'css/WebKitCSSFilterValue.idl',
             'css/WebKitCSSMatrix.idl',
             'css/WebKitCSSMixFunctionValue.idl',
             'css/WebKitCSSRegionRule.idl',
             'css/WebKitCSSTransformValue.idl',
             'dom/Attr.idl',
             'dom/AutocompleteErrorEvent.idl',
             'dom/BeforeLoadEvent.idl',
+            'dom/BeforeUnloadEvent.idl',
             'dom/CDATASection.idl',
             'dom/CharacterData.idl',
             'dom/ClientRect.idl',
             'dom/ClientRectList.idl',
             'dom/Clipboard.idl',
             'dom/Comment.idl',
             'dom/CompositionEvent.idl',
             'dom/CustomEvent.idl',
             'dom/DOMError.idl',
             'dom/DOMException.idl',
@@ skipping 3789 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++"'
               }],
          ],
     }
 }