Updated version number to 3.18 Review URL: https://webrtc-codereview.appspot.com/930027
Updated version number to 3.18
Review URL: https://webrtc-codereview.appspot.com/930027

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6e46d5b1c11a821f9d21a592905278447e96309f

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