Also, change order of definition in fec_test_helper.{h,cc}.
This CL should have no implications on functionality. It is in preparation
for a FlexfecPacketGenerator class, which will be used in the FlexFEC
unit tests.
R=danilchap@webrtc.org
BUG=webrtc:5654
Review-Url: https://codereview.webrtc.org/2276473002
Cr-Commit-Position: refs/heads/master@{#14471}