Slight reduction in string copying for WebRTC stats.
* Only call ToString() for types that require conversion to string. If the value is already of type string, we can just use that value.
* For chrome://webrtc-internals, we can now pass the correct type in most cases (not in64), in the dictionary passed to the web page. This also lowers the overhead of the PeerConnectionTracker when no page requires this data. (I think we can still do better there though).
BUG=webrtc:2822
Review URL: https://codereview.chromium.org/
1009543003
Cr-Commit-Position: refs/heads/master@{#320772}