Auto merge of #12935 - shinglyu:layout_json, r=glennw

Fixed layout flow tree JSON serialization

<!-- Please describe your changes on the following line: -->
The second argument for the `emit_struct()` is the number of fields, if given `0`, the output JSON will always be empty.

This is used in `./mach run -d -Z trace-layout https://servo.org`, which will dump the layout flow tree into a `layout_trace.json` file for debugging.

This also unblocks https://github.com/servo/servo/issues/12675

---
<!-- 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

<!-- Either: -->
- [x] These changes do not require tests because its a debugging tool, not critical for normal code path

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

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12935)
<!-- Reviewable:end -->

components/layout/fragment.rs

@@ -132,7 +132,7 @@ pub struct Fragment {
 
 impl Encodable for Fragment {
     fn encode<S: Encoder>(&self, e: &mut S) -> Result<(), S::Error> {
-        e.emit_struct("fragment", 0, |e| {
+        e.emit_struct("fragment", 3, |e| {
             try!(e.emit_struct_field("id", 0, |e| self.debug_id.encode(e)));
             try!(e.emit_struct_field("border_box", 1, |e| self.border_box.encode(e)));
             e.emit_struct_field("margin", 2, |e| self.margin.encode(e))