[ReactNative] Optimize console.profile and add markers on JS entry points

Summary:
@public

Right now the profiler shows how long the executor took on JS but doesn't show
how long each of the batched calls took, this adds a *very* high level view of JS
execution (still doesn't show properly calls dispatched with setImmediate)

Also added a global property on JS to avoid trips to Native when profiling is
disabled.

Test Plan:
Run the Profiler on any app

{F22491690}

React/Base/RCTProfile.h

@@ -20,6 +20,9 @@
  * before before using it.
  */
 
+NSString *const RCTProfileDidStartProfiling;
+NSString *const RCTProfileDidEndProfiling;
+
 #if RCT_DEV
 
 #define RCTProfileBeginFlowEvent() \

React/Base/RCTProfile.m

@@ -17,6 +17,9 @@
 #import "RCTDefines.h"
 #import "RCTUtils.h"
 
+NSString *const RCTProfileDidStartProfiling = @"RCTProfileDidStartProfiling";
+NSString *const RCTProfileDidEndProfiling = @"RCTProfileDidEndProfiling";
+
 #if RCT_DEV
 
 #pragma mark - Prototypes
@@ -113,10 +116,16 @@ void RCTProfileInit(void)
       RCTProfileSamples: [[NSMutableArray alloc] init],
     };
   );
+
+  [[NSNotificationCenter defaultCenter] postNotificationName:RCTProfileDidStartProfiling
+                                                      object:nil];
 }
 
 NSString *RCTProfileEnd(void)
 {
+  [[NSNotificationCenter defaultCenter] postNotificationName:RCTProfileDidEndProfiling
+                                                      object:nil];
+
   RCTProfileLock(
     NSString *log = RCTJSONStringify(RCTProfileInfo, NULL);
     RCTProfileEventID = 0;