Auto merge of #18550 - nnethercote:bug-1399758-take-2, r=heycam

Measure ImageValue objects (take 2).

We have about 11,500 of these when loading gmail in a Stylo-enabled build, from
SpecifiedUrls; the objects themselves account for about 1.3 MiB of memory, and
the strings within them about 2.9 MiB.

We also have a very small number of them on the Gecko side.

<!-- Please describe your changes on the following line: -->

---
<!-- 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
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because testing is on the Gecko side.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- 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/18550)
<!-- Reviewable:end -->

components/style/gecko/generated/bindings.rs

@@ -1004,6 +1004,10 @@ extern "C" {
                                    aURIString: ServoRawOffsetArc<RustString>)
      -> *mut ImageValue;
 }
+extern "C" {
+    pub fn Gecko_ImageValue_SizeOfIncludingThis(aImageValue: *mut ImageValue)
+     -> usize;
+}
 extern "C" {
     pub fn Gecko_SetLayerImageImageValue(image: *mut nsStyleImage,
                                          aImageValue: *mut ImageValue);