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.

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);