Move btoa() / atob() implementation out of DOMWindow

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 1121 matching lines @@
             'page/DeviceController.h',
             'page/DiagnosticLoggingKeys.cpp',
             'page/DiagnosticLoggingKeys.h',
             'page/DOMPoint.h',
             'page/DOMSecurityPolicy.cpp',
             'page/DOMSelection.cpp',
             'page/DOMSelection.h',
             'page/DOMTimer.cpp',
             'page/DOMTimer.h',
             'page/DOMWindow.cpp',
+            'page/DOMWindow.h',
+            'page/DOMWindowBase64.cpp',
+            'page/DOMWindowBase64.h',
             'page/DOMWindowPagePopup.cpp',
             'page/DOMWindowPagePopup.h',
             'page/DOMWindowProperty.cpp',
             'page/DOMWindowProperty.h',
             'page/DragController.cpp',
             'page/EventHandler.cpp',
             'page/EventSource.cpp',
             'page/EventSource.h',
             'page/FocusController.cpp',
             'page/Frame.cpp',
@@ skipping 2624 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++"'
               }],
          ],
     }
 }