Auto merge of #15813 - canaltinova:will-change, r=upsuper,emilio

Implement parsing/serialization and glue for will-change property

Implement parsing/serialization  and glue for will-change property

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15706 (github issue number if applicable).

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- [x] There are tests for these changes

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components/style/gecko_bindings/bindings.rs

@@ -781,6 +781,17 @@ extern "C" {
                                                       *mut ::std::os::raw::c_void,
                                                   len: usize);
 }
+extern "C" {
+    pub fn Gecko_ClearWillChange(display: *mut nsStyleDisplay, length: usize);
+}
+extern "C" {
+    pub fn Gecko_AppendWillChange(display: *mut nsStyleDisplay,
+                                  atom: *mut nsIAtom);
+}
+extern "C" {
+    pub fn Gecko_CopyWillChangeFrom(dest: *mut nsStyleDisplay,
+                                    src: *mut nsStyleDisplay);
+}
 extern "C" {
     pub fn Gecko_AnimationAppendKeyframe(keyframes:
                                              RawGeckoKeyframeListBorrowedMut,