Prevents RTCP receiver reports, including PLIs with an old receiver-report SSRC, from being dropped from the remote sender's BundleFilter due to no longer being in use. BUG=chromium:523928, webrtc:4883 R=stefan@webrtc.org Review URL: https://codereview.webrtc.org/1404363003 . Cr-Commit-Position: refs/heads/master@{#10359}