use shared_ptr for subscribers vector in event system
use shared_ptr for subscribers vector in event system

Summary:
using shared_ptr for vector of subscribers
Further changes in commit stack support the mutiple subscribers in event system

Reviewed By: davidaurelio

Differential Revision: D15352512

fbshipit-source-id: fac7f4268abf9ca4277734aca2f21cd711eb7d6e

facebook/yoga,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga