WebRtcBrowserTest: Include all stats in getStats.

The only stats dictionary that was missing was 'data-channel'. For it to
be included data channels had to be created. Added CreateDataChannel to
webrtc_browsertest_base.h/cc and peerconnection.js. Unittest updated to
EXPECT that no expected stats are missing.

BUG=chromium:627816

Review-Url: https://codereview.chromium.org/2527273003
Cr-Commit-Position: refs/heads/master@{#434502}