Auto merge of #13126 - splav:HTMLOptionElement.form#13111, r=metajack

Html option element.form#13111

<!-- Please describe your changes on the following line: -->
Add HTMLOptionElement form attribute support

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13111 (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/htmloptionelement.rs

@@ -6,6 +6,7 @@ use dom::attr::Attr;
 use dom::bindings::codegen::Bindings::CharacterDataBinding::CharacterDataMethods;
 use dom::bindings::codegen::Bindings::HTMLOptionElementBinding;
 use dom::bindings::codegen::Bindings::HTMLOptionElementBinding::HTMLOptionElementMethods;
+use dom::bindings::codegen::Bindings::HTMLSelectElementBinding::HTMLSelectElementBinding::HTMLSelectElementMethods;
 use dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
 use dom::bindings::inheritance::Castable;
 use dom::bindings::js::Root;
@@ -14,6 +15,8 @@ use dom::characterdata::CharacterData;
 use dom::document::Document;
 use dom::element::{AttributeMutation, Element};
 use dom::htmlelement::HTMLElement;
+use dom::htmlformelement::HTMLFormElement;
+use dom::htmloptgroupelement::HTMLOptGroupElement;
 use dom::htmlscriptelement::HTMLScriptElement;
 use dom::htmlselectelement::HTMLSelectElement;
 use dom::node::{Node, UnbindContext};
@@ -110,6 +113,19 @@ impl HTMLOptionElementMethods for HTMLOptionElement {
         self.upcast::<Node>().SetTextContent(Some(value))
     }
 
+    // https://html.spec.whatwg.org/multipage/#dom-option-form
+    fn GetForm(&self) -> Option<Root<HTMLFormElement>> {
+        let parent = self.upcast::<Node>().GetParentNode().and_then(|p|
+            if p.is::<HTMLOptGroupElement>() {
+                p.upcast::<Node>().GetParentNode()
+            } else {
+                Some(p)
+            }
+        );
+
+        parent.and_then(|p| p.downcast::<HTMLSelectElement>().and_then(|s| s.GetForm()))
+    }
+
     // https://html.spec.whatwg.org/multipage/#attr-option-value
     fn Value(&self) -> DOMString {
         let element = self.upcast::<Element>();

components/script/dom/webidls/HTMLOptionElement.webidl

@@ -9,7 +9,7 @@
 [Exposed=(Window,Worker)]
 interface HTMLOptionElement : HTMLElement {
              attribute boolean disabled;
-  //readonly attribute HTMLFormElement? form;
+             readonly attribute HTMLFormElement? form;
              attribute DOMString label;
              attribute boolean defaultSelected;
              attribute boolean selected;