Fix "unreachable code" warnings (MSVC warning 4702) in Blink.

Source/build/scripts/make_css_value_keywords.py

Index: Source/build/scripts/make_css_value_keywords.py
===================================================================
--- Source/build/scripts/make_css_value_keywords.py	(revision 169502)
+++ Source/build/scripts/make_css_value_keywords.py	(working copy)
@@ -86,13 +86,9 @@
 
 bool isValueAllowedInMode(unsigned short id, CSSParserMode mode)
 {
-    // FIXME: Investigate whether we can deprecate the former
-    // two as only QuirksOrUASheet is used in CSSValueKeyword.in
     switch (id) {
         %(ua_sheet_mode_values_keywords)s
             return isUASheetBehavior(mode);
-        %(quirks_mode_values_keywords)s
-            return isQuirksModeBehavior(mode);
         %(quirks_mode_or_ua_sheet_mode_values_keywords)s
             return isUASheetBehavior(mode) || isQuirksModeBehavior(mode);
         default:
@@ -164,7 +160,6 @@
             'value_keyword_offsets': '\n'.join(map(lambda offset: '  %d,' % offset, keyword_offsets)),
             'value_keyword_to_enum_map': '\n'.join(map(lambda property: '%(name)s, %(enum_name)s' % property, self._value_keywords)),
             'ua_sheet_mode_values_keywords': '\n        '.join(map(self._case_value_keyword, self._value_keywords_with_mode('UASheet'))),
-            'quirks_mode_values_keywords': '\n        '.join(map(self._case_value_keyword, self._value_keywords_with_mode('Quirks'))),
             'quirks_mode_or_ua_sheet_mode_values_keywords': '\n    '.join(map(self._case_value_keyword, self._value_keywords_with_mode('QuirksOrUASheet'))),
         }
         # FIXME: If we could depend on Python 2.7, we would use subprocess.check_output