[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}

Libraries/Utilities/BridgeProfiling.js

@@ -0,0 +1,37 @@
+/**
+ * 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 BridgeProfiling
+ * @flow
+ */
+'use strict';
+
+var GLOBAL = GLOBAL || this;
+
+var BridgeProfiling = {
+  profile(profileName: String, args?: any) {
+    if (GLOBAL.__BridgeProfilingIsProfiling) {
+      if (args) {
+        try {
+          args = JSON.stringify(args);
+        } catch(err) {
+          args = err.message;
+        }
+      }
+      console.profile(profileName, args);
+    }
+  },
+
+  profileEnd() {
+    if (GLOBAL.__BridgeProfilingIsProfiling) {
+      console.profileEnd();
+    }
+  },
+};
+
+module.exports = BridgeProfiling;