mutex lock while accessing event subscribers vector for thread safety
mutex lock while accessing event subscribers vector for thread safety

Summary: Using Mutex lock_guard mechanism when writing to subscribers and when accessing them in publish to make a copy

Reviewed By: davidaurelio

Differential Revision: D15391679

fbshipit-source-id: 16713ff28ce1762a5ca4c48c152897a92417e80b

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