Various memory measurement improvements (#36834)

The two significant changes here are 1) a commit that frees memory used
to perform memory reporting once the reporting is complete, 2) memory
reporting for the system font service. There are various other commits
that remove `#[ignore_malloc_size_of]` attributes for data that we are
now able to measure, but they do not significantly change our
measurements when testing servo.org.

Testing: Comparing the output of about:memory on servo.org.

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/url/lib.rs

@@ -35,7 +35,7 @@ pub enum UrlError {
 }
 
 #[derive(Clone, Deserialize, Eq, Hash, MallocSizeOf, Ord, PartialEq, PartialOrd, Serialize)]
-pub struct ServoUrl(#[ignore_malloc_size_of = "Arc"] Arc<Url>);
+pub struct ServoUrl(#[conditional_malloc_size_of] Arc<Url>);
 
 impl ServoUrl {
     pub fn from_url(url: Url) -> Self {