Merge SignalPortPruned and SignalPortsRemoved. These two signals have the same purpose and is kind of redundant. Rename to SignalPortsPruned. BUG= R=pthatcher@webrtc.org, zhihuang@webrtc.org Review URL: https://codereview.webrtc.org/2176743003 . Cr-Original-Commit-Position: refs/heads/master@{#13562} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: 8eeecabd3345c0f5bd86d7a32d0b57268179bebf