Adding Stop method of periodic tasks in SendSideCongestionController (SSCC). This is utilized in a later CL enabling switching the network controller which requires stopping the old periodic task and starting a new one with a new update period. Bug: webrtc:8415 Change-Id: I2e56c1e1fe10d88c038b2f290d94c08723ddf4e4 Reviewed-on: https://webrtc-review.googlesource.com/67280 Commit-Queue: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Niels Moller <nisse@webrtc.org> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org> Cr-Commit-Position: refs/heads/master@{#22786}