[ReactNative] Add PerformanceLogger to measure TTI

Summary:
@public

Add PerformanceLogger to keep track of JS download, initial script execution and
full TTI.

Test Plan:
The Native side currently calls `addTimespans` when it's finish initializing
with the six values (start and end for the three events), so I just checked it
with a `PerformanceLogger.logTimespans()` at the end of the function.

```
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "ScriptDownload: 48ms"
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "ScriptExecution: 106ms"
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "TTI: 293ms"
```

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -24,6 +24,7 @@
 
 // Just to make sure the JS gets packaged up.
 require('RCTDeviceEventEmitter');
+require('PerformanceLogger');
 
 if (typeof GLOBAL === 'undefined') {
   GLOBAL = this;

Libraries/Utilities/PerformanceLogger.js

@@ -0,0 +1,96 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule PerformanceLogger
+ */
+'use strict';
+
+
+var performanceNow = require('performanceNow');
+
+var timespans = {};
+
+/**
+ * This is meant to collect and log performance data in production, which means
+ * it needs to have minimal overhead.
+ */
+var PerformanceLogger = {
+  addTimespan(key, lengthInMs, description) {
+    if (timespans[key]) {
+      if (__DEV__) {
+        console.log(
+          'PerformanceLogger: Attempting to add a timespan that already exists'
+        );
+      }
+      return;
+    }
+
+    timespans[key] = {
+      description: description,
+      totalTime: lengthInMs,
+    };
+  },
+
+  startTimespan(key, description) {
+    if (timespans[key]) {
+      if (__DEV__) {
+        console.log(
+          'PerformanceLogger: Attempting to start a timespan that already exists'
+        );
+      }
+      return;
+    }
+
+    timespans[key] = {
+      description: description,
+      startTime: performanceNow(),
+    };
+  },
+
+  stopTimespan(key) {
+    if (!timespans[key] || !timespans[key].startTime) {
+      if (__DEV__) {
+        console.log(
+          'PerformanceLogger: Attempting to end a timespan that has not started'
+        );
+      }
+      return;
+    }
+
+    timespans[key].endTime = performanceNow();
+    timespans[key].totalTime =
+      timespans[key].endTime - timespans[key].startTime;
+  },
+
+  clearTimespans() {
+    timespans = {};
+  },
+
+  getTimespans() {
+    return timespans;
+  },
+
+  logTimespans() {
+    for (var key in timespans) {
+      console.log(key + ': ' + timespans[key].totalTime + 'ms');
+    }
+  },
+
+  addTimespans(newTimespans, labels) {
+    for (var i = 0, l = newTimespans.length; i < l; i += 2) {
+      var label = labels[i / 2];
+      PerformanceLogger.addTimespan(
+        label,
+        (newTimespans[i + 1] - newTimespans[i]),
+        label
+      );
+    }
+  }
+};
+
+module.exports = PerformanceLogger;