RTCIceCandidateStats.deleted = false added.

Since previously, only the deleted = false case is supported, but now
that stat is added. It's a recent addition to the spec:
https://w3c.github.io/webrtc-stats/#dom-rtcicecandidatestats-deleted

BUG=webrtc:6756, chromium:632723, chromium:627816

Review-Url: https://codereview.webrtc.org/2591963003
Cr-Commit-Position: refs/heads/master@{#15871}

webrtc/api/stats/rtcstats_objects.h

@@ -176,6 +176,9 @@ class RTCIceCandidateStats : public RTCStats {
   RTCStatsMember<int32_t> priority;
   // TODO(hbos): Not collected by |RTCStatsCollector|. crbug.com/632723
   RTCStatsMember<std::string> url;
+  // TODO(hbos): |deleted = true| case is not supported by |RTCStatsCollector|.
+  // crbug.com/632723
+  RTCStatsMember<bool> deleted;  // = false
 
  protected:
   RTCIceCandidateStats(