Fix a compilation problem with ObservingHandler.
Fix a compilation problem with ObservingHandler.

Summary:
Got a compilation error when trying to use ObservingPipelineFactory.
Fixing this.

Test Plan: Tried using ObservingPipelineFactory in an example.

Reviewed By: davejwatson@fb.com

Subscribers: fugalh, jsedgwick, praveenr, afrind, noamler

FB internal diff: D2209586

Signature: t1:2209586:1435778732:32a1b133da7beb256f4129a29dfe540fa53eb10a

AresTao/wangle,Orvid/wangle,raphaelamorim/wangle,Orvid/wangle,orian/wangle,jamperry/wangle,jamperry/wangle,mzhaom/wangle,facebook/wangle,rklabs/wangle,raphaelamorim/wangle,AresTao/wangle,Dalzhim/wangle,facebook/wangle,mzhaom/wangle,orian/wangle,rklabs/wangle,Dalzhim/wangle,facebook/wangle