IDL parser: Drop accepting extended attributes on enum values

For now, IDL parser allows to put extended attributes on enum values,
that is syntactically invalid in Web IDL spec [1].
In Blink, other IDL compiler parts ignore them, and no IDL files have them.

In order to avoid misleading developers, this CL makes IDL parser
to disallow extended attributes on enum values.

[1] https://heycam.github.io/webidl/#idl-enums


Bug: 871867
Change-Id: Ic57406ce3ff94a0c1a4cf38da12b217c05667eb0
Reviewed-on: https://chromium-review.googlesource.com/1169482Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582009}

tools/idl_parser/idl_parser.py

@@ -464,9 +464,9 @@ class IDLParser(object):
     p[0] = self.BuildError(p, 'Enum')
 
   def p_EnumValueList(self, p):
-    """EnumValueList : ExtendedAttributeList string EnumValueListComma"""
-    enum = self.BuildNamed('EnumItem', p, 2, p[1])
-    p[0] = ListFromConcat(enum, p[3])
+    """EnumValueList : string EnumValueListComma"""
+    enum = self.BuildNamed('EnumItem', p, 1)
+    p[0] = ListFromConcat(enum, p[2])
 
   def p_EnumValueListComma(self, p):
     """EnumValueListComma : ',' EnumValueListString
@@ -475,11 +475,11 @@ class IDLParser(object):
       p[0] = p[2]
 
   def p_EnumValueListString(self, p):
-    """EnumValueListString : ExtendedAttributeList string EnumValueListComma
+    """EnumValueListString : string EnumValueListComma
                            |"""
     if len(p) > 1:
-      enum = self.BuildNamed('EnumItem', p, 2, p[1])
-      p[0] = ListFromConcat(enum, p[3])
+      enum = self.BuildNamed('EnumItem', p, 1)
+      p[0] = ListFromConcat(enum, p[2])
 
   def p_CallbackRest(self, p):
     """CallbackRest : identifier '=' ReturnType '(' ArgumentList ')' ';'"""