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/profile_traits/mem.rs

@@ -8,6 +8,7 @@
 
 use ipc_channel::ipc::{self, IpcSender};
 use ipc_channel::router::ROUTER;
+use serde;
 use std::marker::Send;
 use std::sync::mpsc::Sender;
 
@@ -25,6 +26,14 @@ impl<T> OpaqueSender<T> for Sender<T> {
     }
 }
 
+impl<T> OpaqueSender<T> for IpcSender<T> where T: serde::Serialize {
+    fn send(&self, message: T) {
+        if let Err(e) = IpcSender::send(self, message) {
+            warn!("Error communicating with the target thread from the profiler: {}", e);
+        }
+    }
+}
+
 /// Front-end representation of the profiler used to communicate with the
 /// profiler.
 #[derive(Clone, Deserialize, Serialize)]