Update webidl.py from upstream

pref-main-thread.patch

@@ -0,0 +1,27 @@
+--- WebIDL.py
++++ WebIDL.py
+@@ -1362,12 +1362,6 @@ class IDLInterfaceOrNamespace(IDLObjectWithScope, IDLExposureMixins):
+                 for bindingAlias in member.bindingAliases:
+                     checkDuplicateNames(member, bindingAlias, "BindingAlias")
+ 
+-
+-        if self.getExtendedAttribute("Pref") and self.isExposedOffMainThread():
+-            raise WebIDLError("[Pref] used on an interface that is not "
+-                              "main-thread-only",
+-                              [self.location])
+-
+         # Conditional exposure makes no sense for interfaces with no
+         # interface object, unless they're navigator properties.
+         # And SecureContext makes sense for interfaces with no interface object,
+@@ -3619,11 +3613,6 @@ class IDLInterfaceMember(IDLObjectWithIdentifier, IDLExposureMixins):
+         IDLExposureMixins.finish(self, scope)
+ 
+     def validate(self):
+-        if self.getExtendedAttribute("Pref") and self.isExposedOffMainThread():
+-            raise WebIDLError("[Pref] used on an interface member that is not "
+-                              "main-thread-only",
+-                              [self.location])
+-
+         if self.isAttr() or self.isMethod():
+             if self.affects == "Everything" and self.dependsOn != "Everything":
+                 raise WebIDLError("Interface member is flagged as affecting "