Add a flag to store perf results as a JSON file.
Add a flag to store perf results as a JSON file.

Add a flag to store perf results as a JSON file in the format specified
by https://github.com/catapult-project/catapult/blob/master/dashboard/docs/data-format.md

Bug: webrtc:7156
Change-Id: Ia5b0317f0f5dc8767fa219f42bc39bf4073203e8
Reviewed-on: https://webrtc-review.googlesource.com/29160
Reviewed-by: Patrik Höglund <b486136ee68458b09c92c86c39f451c87d6ef6a1@webrtc.org>
Commit-Queue: Edward Lemur <d05609f027dc667cc37492b1dc1bdea5d60fa82a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#21082}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc