servo: Merge #11404 - Add Servo_RestyleSubtree (from heycam:restyle-subtree); r=bholley

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 --faster` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because geckolib-only

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

----

See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1275452

r? @bholley

Source-Repo: https://github.com/servo/servo
Source-Revision: 0293a3df16384b84ecbfee2838ffd87ca6ed4a01

servo/ports/geckolib/gecko_bindings/bindings.rs

@@ -223,6 +223,8 @@ extern "C" {
     pub fn Servo_Initialize();
     pub fn Servo_RestyleDocument(doc: *mut RawGeckoDocument,
                                  set: *mut RawServoStyleSet);
+    pub fn Servo_RestyleSubtree(node: *mut RawGeckoNode,
+                                set: *mut RawServoStyleSet);
     pub fn Gecko_GetAttrAsUTF8(element: *mut RawGeckoElement,
                                ns: *mut nsIAtom, name: *mut nsIAtom,
                                length: *mut u32)

servo/ports/geckolib/glue.rs

@@ -73,13 +73,9 @@ pub extern "C" fn Servo_Initialize() -> () {
     env_logger::init().unwrap();
 }
 
-#[no_mangle]
-pub extern "C" fn Servo_RestyleDocument(doc: *mut RawGeckoDocument, raw_data: *mut RawServoStyleSet) -> () {
-    let document = unsafe { GeckoDocument::from_raw(doc) };
-    let node = match document.root_node() {
-        Some(x) => x,
-        None => return,
-    };
+fn restyle_subtree(node: GeckoNode, raw_data: *mut RawServoStyleSet) {
+    debug_assert!(node.is_element() || node.is_text_node());
+
     let data = unsafe { &mut *(raw_data as *mut PerDocumentStyleData) };
 
     // Force the creation of our lazily-constructed initial computed values on
@@ -112,6 +108,23 @@ pub extern "C" fn Servo_RestyleDocument(doc: *mut RawGeckoDocument, raw_data: *m
     }
 }
 
+#[no_mangle]
+pub extern "C" fn Servo_RestyleSubtree(node: *mut RawGeckoNode,
+                                       raw_data: *mut RawServoStyleSet) -> () {
+    let node = unsafe { GeckoNode::from_raw(node) };
+    restyle_subtree(node, raw_data);
+}
+
+#[no_mangle]
+pub extern "C" fn Servo_RestyleDocument(doc: *mut RawGeckoDocument, raw_data: *mut RawServoStyleSet) -> () {
+    let document = unsafe { GeckoDocument::from_raw(doc) };
+    let node = match document.root_node() {
+        Some(x) => x,
+        None => return,
+    };
+    restyle_subtree(node, raw_data);
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_DropNodeData(data: *mut ServoNodeData) -> () {
     unsafe {