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/fonts/tests/font_context.rs

@@ -137,6 +137,7 @@ mod font_context {
                         break;
                     },
                     SystemFontServiceMessage::Ping => {},
+                    SystemFontServiceMessage::CollectMemoryReport(..) => {},
                 }
             }
         }