Add a facility for automated performance measurement

We want to be able to summarize the behavior of the shell's
performance in a series of "metrics", like the latency between
clicking on the Activities button and seeing a response.

This patch adds the ability to create a script under perf/
in a special format that automates a series of actions in the
shell, writing events to the performance log, then collects
statistics as the log as replayed and turns them into a set
of metrics.

The script is then executed by running as gnome-shell
--perf=<script>.

The 'core' script which is added here will be the primary
performance measurement script that we use for shell performance
regression testing; right now it has a couple of placeholder
metrics.

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

js/perf/core.js

@@ -0,0 +1,67 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+const Main = imports.ui.main;
+const Scripting = imports.ui.scripting;
+
+// This performance script measure the most important (core) performance
+// metrics for the shell. By looking at the output metrics of this script
+// someone should be able to get an idea of how well the shell is performing
+// on a particular system.
+
+let METRIC_DESCRIPTIONS = {
+    overviewLatencyFirst: "Time to first frame after triggering overview, first time",
+    overviewFramesFirst: "Frames displayed when going to overview, first time",
+    overviewLatencySubsequent: "Time to first frame after triggering overview, second time",
+    overviewFramesSubsequent: "Frames displayed when going to overview, second time"
+};
+
+let METRICS = {
+};
+
+function run() {
+    Scripting.defineScriptEvent("overviewShowStart", "Starting to show the overview");
+    Scripting.defineScriptEvent("overviewShowDone", "Overview finished showing");
+
+    yield Scripting.sleep(1000);
+    yield Scripting.waitLeisure();
+    for (let i = 0; i < 2; i++) {
+        Scripting.scriptEvent('overviewShowStart');
+        Main.overview.show();
+        yield Scripting.waitLeisure();
+        Scripting.scriptEvent('overviewShowDone');
+        Main.overview.hide();
+        yield Scripting.waitLeisure();
+    }
+}
+
+let showingOverview = false;
+let overviewShowStart;
+let overviewFrames;
+let overviewLatency;
+
+function script_overviewShowStart(time) {
+    showingOverview = true;
+    overviewShowStart = time;
+    overviewFrames = 0;
+}
+
+function script_overviewShowDone(time) {
+    showingOverview = false;
+
+    if (!('overviewLatencyFirst' in METRICS)) {
+        METRICS.overviewLatencyFirst = overviewLatency;
+        METRICS.overviewFramesFirst = overviewFrames;
+    } else {
+        METRICS.overviewLatencySubsequent = overviewLatency;
+        METRICS.overviewFramesSubsequent = overviewFrames;
+    }
+}
+
+function clutter_stagePaintDone(time) {
+    if (showingOverview) {
+        if (overviewFrames == 0)
+            overviewLatency = time - overviewShowStart;
+
+        overviewFrames++;
+    }
+}