Bug 1385478 - Part 2: Change Required/IsRequired() to look at NS_EVENT_STATE_REQUIRED instead. r=bz

In order to speed up Required/IsRequired(), instead of querying for the
@required attribute, we're now using the NS_EVENT_STATE_REQUIRED flag to know
whether an element's value is required.

For this to work correctly, we need to set NS_EVENT_STATE_REQUIRED earlier,
that is, in AfterSetAttr(), before any consumer of IsRequired(). We also need
to update or clear our required states when input type changes, since we may
have changed from a required input type to a non-required input type or
vice versa.

Note that NS_EVENT_STATE_REQUIRED/OPTIONAL is now part of the
EXTERNALLY_MANAGED_STATES.

MozReview-Commit-ID: Bjiby9GqJSB

dom/html/HTMLSelectElement.cpp

@@ -1333,6 +1333,11 @@ HTMLSelectElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
       UpdateValueMissingValidityState();
       UpdateBarredFromConstraintValidation();
     } else if (aName == nsGkAtoms::required) {
+      // This *has* to be called *before* UpdateValueMissingValidityState
+      // because UpdateValueMissingValidityState depends on our required
+      // state.
+      UpdateRequiredState(!!aValue, aNotify);
+
       UpdateValueMissingValidityState();
     } else if (aName == nsGkAtoms::autocomplete) {
       // Clear the cached @autocomplete attribute and autocompleteInfo state.
@@ -1530,12 +1535,6 @@ HTMLSelectElement::IntrinsicState() const
     }
   }
 
-  if (Required()) {
-    state |= NS_EVENT_STATE_REQUIRED;
-  } else {
-    state |= NS_EVENT_STATE_OPTIONAL;
-  }
-
   return state;
 }