Make frame-timestamp logging optional

Instead of always logging frame timestamps for every frame - which
was using >26 bytes of memory per frame, or 5MB per hour of continuous
redrawing - make frame timestamps something that defaults off and is
turned turned on using a new ShellGlobal::frame-timestamps property by
the perf scripts.

https://bugzilla.gnome.org/show_bug.cgi?id=732350

js/perf/core.js

@@ -72,6 +72,9 @@ function run() {
     Scripting.defineScriptEvent("applicationsShowStart", "Starting to switch to applications view");
     Scripting.defineScriptEvent("applicationsShowDone", "Done switching to applications view");
 
+    // Enable recording of timestamps for different points in the frame cycle
+    global.frame_timestamps = true;
+
     Main.overview.connect('shown', function() {
                               Scripting.scriptEvent('overviewShowDone');
                           });