Avoid race in Call destructor
Avoid race in Call destructor

Don't update histograms until we're sure process threads won't call into
the instance being destructed, trying to update stats.

BUG=webrtc:6103

Review-Url: https://codereview.webrtc.org/2151433002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13461}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc