Refactor performance memory API implementation

Summary:
This diff refactors performance memory API and the RN Tester example.

- The returned value from C++ is number, so no need to cast
- Reuse `MemoryInfo` in RNTester example

Changelog:
[General][Internal] - Code refactor for performance memory API implementation

Reviewed By: rubennorte

Differential Revision: D43523878

fbshipit-source-id: 37d1f6a829a8eac45f7e3791ad36be0c199c6041

Libraries/WebPerformance/MemoryInfo.js

@@ -31,14 +31,23 @@ export default class MemoryInfo {
     }
   }
 
+  /**
+   * The maximum size of the heap, in bytes, that is available to the context
+   */
   get jsHeapSizeLimit(): ?number {
     return this._jsHeapSizeLimit;
   }
 
+  /**
+   * The total allocated heap size, in bytes
+   */
   get totalJSHeapSize(): ?number {
     return this._totalJSHeapSize;
   }
 
+  /**
+   * The currently active segment of JS heap, in bytes.
+   */
   get usedJSHeapSize(): ?number {
     return this._usedJSHeapSize;
   }