Add a freeze metric to the video quality test.
Add a freeze metric to the video quality test.

Defined as time between freezes (nothing rendered for 150 ms).

Bug: webrtc:8861
Change-Id: I56eae3beb7278b6d1894a0593ae3092c9f3cb1cc
Reviewed-on: https://webrtc-review.googlesource.com/49780
Reviewed-by: Erik Språng <3ab8c90c99d6dd7822e0fad803ab0211dd287ae5@webrtc.org>
Reviewed-by: Rasmus Brandt <9545de3a34b925a736ae2b5ee593f3033f743a28@webrtc.org>
Commit-Queue: Stefan Holmer <eb0877843acc39f8ef6f7269937dee931c372d23@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#21970}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc