Create a method to calculate and show the frame rate in WebCL examples

(Examples/WebCL)
issue #72

    Created a method to show the frame rate. Developer uses
    Metrics.SHOW_FPS constant to show or not. Default is false.

    Code style reviewed.

    Reviewed by NOBODY (OOPS!).

    * WebCL/Random/index.html:
    * WebCL/libs/Metrics.js: