RTCPeerConnectionStats: Removed fixed TODO comments.

I forget to remove these when fixing them.

BUG=chromium:636818
NOTRY=True
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2522023003
Cr-Original-Commit-Position: refs/heads/master@{#15215}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 42eee12614c8b931f416c9830dc247eba85e73b1