Add simple malloc statistics and metrics

Add some basic statistics for allocated memory based on mallinfo(),
and use that to define two metrics:

 usedAfterOverview: bytes used after the overview is shown once
 leakedAfterOverview: additional bytes used when the overview is
   shown a second time.

https://bugzilla.gnome.org/show_bug.cgi?id=618189

js/ui/scripting.js

@@ -94,6 +94,15 @@ function scriptEvent(name) {
     Shell.PerfLog.get_default().event("script." + name);
 }
 
+/**
+ * collectStatistics
+ *
+ * Convenience function to trigger statistics collection
+ */
+function collectStatistics() {
+    Shell.PerfLog.get_default().collect_statistics();
+}
+
 function _step(g, finish, onError) {
     try {
         let waitFunction = g.next();