RTCStats operator== bugfix

a == b would return true if a.member is defined even if b.member is
undefined if their values were equal. We would say that b does not have
a value in that case but its value_ member would still be initialized to
something that is being compared to. Bugfix makes sure not to do value
comparison in this case if b is undefined.

BUG=chromium:627816

Review-Url: https://codereview.webrtc.org/2517163002
Cr-Commit-Position: refs/heads/master@{#15172}