Disable applicable declarations cache for elements with a style attribute.

The cache is never effective in this case since a style attribute
adds to the set of applicable declarations.

This also avoid avoids the cache giving out-dated (incorrect) results
when `PropertyDeclarationBlock` becomes mutable in a later commit.
This is sufficient for now since CSSOM is only implemented for
`Element.style`. When StyleRule mutation is added,
some additional mechanism (such as evicting the cache) will be needed.

components/style/matching.rs

@@ -502,7 +502,13 @@ trait PrivateMatchMethods: TNode {
                                             -> Arc<ComputedValues>
         where Ctx: StyleContext<'a>
     {
-        let mut cacheable = true;
+        // Don’t cache applicable declarations for elements with a style attribute.
+        // Since the style attribute contributes to that set, no other element would have the same set
+        // and the cache would not be effective anyway.
+        // This also works around the test failures at
+        // https://github.com/servo/servo/pull/13459#issuecomment-250717584
+        let has_style_attribute = self.as_element().map_or(false, |e| e.style_attribute().is_some());
+        let mut cacheable = !has_style_attribute;
         let shared_context = context.shared_context();
         if animate_properties {
             cacheable = !self.update_animations_for_cascade(shared_context,