Measure cache memory usage (#19251):

Made the memory cache data structure derive MallocSizeOf, along with
manual size_of() implementations in malloc_size_of.

Added a Measurable struct that acts as a container for fields size_of() can be called for.

Added a new IpcReceiver used for listening to messages from the memory profiler,
and used run_with_memory reporting to register a memory reporter in the thread.
Now when a message from the memory profiler arrives, report includes sizes of public and private http caches.

Updated test file.

components/servo/lib.rs

@@ -462,6 +462,7 @@ fn create_constellation(user_agent: Cow<'static, str>,
         new_resource_threads(user_agent,
                              devtools_chan.clone(),
                              time_profiler_chan.clone(),
+                             mem_profiler_chan.clone(),
                              config_dir);
     let font_cache_thread = FontCacheThread::new(public_resource_threads.sender(),
                                                  webrender_api_sender.create_api());