Added NiceMock for MockRtcEventLog in several unittests where we don't care about the event logging. BUG=webrtc:4741 R=stefan@webrtc.org Review URL: https://codereview.webrtc.org/2121993002 . Cr-Original-Commit-Position: refs/heads/master@{#13388} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: fa1d568730ed5dedbed840df0836d1a2fd7a19ee