[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"
```

React/Base/RCTBridge.m

@@ -20,6 +20,7 @@
 #import "RCTKeyCommands.h"
 #import "RCTLog.h"
 #import "RCTPerfStats.h"
+#import "RCTPerformanceLogger.h"
 #import "RCTProfile.h"
 #import "RCTRedBox.h"
 #import "RCTRootView.h"
@@ -612,6 +613,8 @@ dispatch_queue_t RCTJSThread;
   RCTAssertMainThread();
 
   if ((self = [super init])) {
+    RCTPerformanceLoggerStart(RCTPLTTI);
+
     _bundleURL = bundleURL;
     _moduleProvider = block;
     _launchOptions = [launchOptions copy];
@@ -981,8 +984,10 @@ RCT_INNER_BRIDGE_ONLY(_invokeAndProcessModule:(__unused NSString *)module
   } else {
 
     RCTProfileBeginEvent();
+    RCTPerformanceLoggerStart(RCTPLScriptDownload);
     RCTJavaScriptLoader *loader = [[RCTJavaScriptLoader alloc] initWithBridge:self];
     [loader loadBundleAtURL:bundleURL onComplete:^(NSError *error, NSString *script) {
+      RCTPerformanceLoggerEnd(RCTPLScriptDownload);
       RCTProfileEndEvent(@"JavaScript dowload", @"init,download", @[]);
 
       _loading = NO;