Auto merge of #15837 - upsuper:bug1344135, r=emilio

Return true in set_property only when declaration block is changed

This is [bug 1344135](https://bugzilla.mozilla.org/show_bug.cgi?id=1344135).

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components/script/dom/cssstyledeclaration.rs

@@ -266,11 +266,9 @@ impl CSSStyleDeclaration {
 
             // Step 8
             // Step 9
-            // We could try to be better I guess?
-            *changed = !declarations.is_empty();
+            *changed = false;
             for declaration in declarations {
-                // TODO(emilio): We could check it changed
-                pdb.set_parsed_declaration(declaration.0, importance);
+                *changed |= pdb.set_parsed_declaration(declaration.0, importance);
             }
 
             Ok(())