Auto merge of #14535 - servo:property-id, r=mbrubeck

Introduce a PropertyId enum and use it instead of strings of property names

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* `LonghandId` and `ShorthandId` are C-like enums
* `Atom` is used for the name of custom properties.
* `PropertyDeclarationId` is the identifier for `PropertyDeclaration`,
  after parsing and shorthand expansion. (Longhand or custom property.)
* `PropertyId` represents any CSS property, e.g. in CSSOM.
  (Longhand, shorthand, or custom.)

CC @upsuper

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

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

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components/layout_thread/lib.rs

@@ -466,7 +466,7 @@ impl LayoutThread {
                     scroll_root_id_response: None,
                     scroll_area_response: Rect::zero(),
                     overflow_response: NodeOverflowResponse(None),
-                    resolved_style_response: None,
+                    resolved_style_response: String::new(),
                     offset_parent_response: OffsetParentResponse::empty(),
                     margin_style_response: MarginStyleResponse::empty(),
                     stacking_context_scroll_offsets: HashMap::new(),
@@ -1018,7 +1018,7 @@ impl LayoutThread {
                         rw_data.scroll_root_id_response = None;
                     },
                     ReflowQueryType::ResolvedStyleQuery(_, _, _) => {
-                        rw_data.resolved_style_response = None;
+                        rw_data.resolved_style_response = String::new();
                     },
                     ReflowQueryType::OffsetParentQuery(_) => {
                         rw_data.offset_parent_response = OffsetParentResponse::empty();