[ReactNative] Add more markers and fix FPS graph

Summary:
@public

Add marker to show JavaScript download duration + flow arrows to show the origin
of the UI blocks being flushed.
Also fixed the condition on `RCTPerfStats`, UI and JS graphs were being created
at startup time, now they're just created on the first time they're shown.

Test Plan:
The markers:

{F22577660}

To check the FPS graph, enable it on the DevMenu, and it should appear initially
empty, instead of previously filled as before.

React/Base/RCTBridge.m

@@ -951,8 +951,10 @@ RCT_INNER_BRIDGE_ONLY(_invokeAndProcessModule:(__unused NSString *)module
     });
   } else {
 
+    RCTProfileBeginEvent();
     RCTJavaScriptLoader *loader = [[RCTJavaScriptLoader alloc] initWithBridge:self];
     [loader loadBundleAtURL:bundleURL onComplete:^(NSError *error, NSString *script) {
+      RCTProfileEndEvent(@"JavaScript dowload", @"init,download", @[]);
 
       _loading = NO;
       if (!self.isValid) {
@@ -1119,12 +1121,11 @@ RCT_INNER_BRIDGE_ONLY(_invokeAndProcessModule:(__unused NSString *)module
 {
   RCTAssert(onComplete != nil, @"onComplete block passed in should be non-nil");
 
-  RCTProfileBeginEvent();
-
+  RCTProfileBeginFlowEvent();
   [_javaScriptExecutor executeApplicationScript:script sourceURL:url onComplete:^(NSError *scriptLoadError) {
+    RCTProfileEndFlowEvent();
     RCTAssertJSThread();
 
-    RCTProfileEndEvent(@"ApplicationScript", @"js_call,init", scriptLoadError);
     if (scriptLoadError) {
       onComplete(scriptLoadError);
       return;

React/Base/RCTPerfStats.m

@@ -41,7 +41,7 @@ RCT_EXPORT_MODULE()
 
 - (RCTFPSGraph *)jsGraph
 {
-  if (!_jsGraph) {
+  if (!_jsGraph && _container) {
     UIColor *jsColor = [UIColor colorWithRed:0 green:1 blue:0 alpha:1];
     _jsGraph = [[RCTFPSGraph alloc] initWithFrame:CGRectMake(2, 2, 124, 34)
                                     graphPosition:RCTFPSGraphPositionRight
@@ -54,7 +54,7 @@ RCT_EXPORT_MODULE()
 
 - (RCTFPSGraph *)uiGraph
 {
-  if (!_uiGraph) {
+  if (!_uiGraph && _container) {
     UIColor *uiColor = [UIColor colorWithRed:0 green:1 blue:1 alpha:1];
     _uiGraph = [[RCTFPSGraph alloc] initWithFrame:CGRectMake(2, 2, 124, 34)
                                     graphPosition:RCTFPSGraphPositionLeft

React/Modules/RCTUIManager.m

@@ -979,7 +979,9 @@ RCT_EXPORT_METHOD(findSubviewIn:(NSNumber *)reactTag atPoint:(CGPoint)point call
   [_pendingUIBlocksLock unlock];
 
   // Execute the previously queued UI blocks
+  RCTProfileBeginFlowEvent();
   dispatch_async(dispatch_get_main_queue(), ^{
+    RCTProfileEndFlowEvent();
     RCTProfileBeginEvent();
     for (dispatch_block_t block in previousPendingUIBlocks) {
       block();