Reland "iOS: Save perf results under Documents/perf_result.json"

This will require a manual roll to downstream projects, since
the //test:perf_test target was introduced.

This is a reland of 10a8e7a9b5261a7e3ce19900ba3511be3b5911f8
Original change's description:
> iOS: Save perf results under Documents/perf_result.json
>
> TBR=henrika@webrtc.org
>
> Bug: webrtc:7156
> Change-Id: Ib00992cce0007e0b5c9274340df1a892f810b0c5
> Reviewed-on: https://webrtc-review.googlesource.com/29202
> Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
> Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
> Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21244}

R=henrika@webrtc.org, phoglund@webrtc.org

Bug: webrtc:7156
Change-Id: I85fc7bc5fce0894af90017b71b9952b61b523424
Reviewed-on: https://webrtc-review.googlesource.com/37643
Reviewed-by: Patrik Höglund <phoglund@google.com>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21518}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 3c4f18e..552bfb4 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -118,6 +118,7 @@
       "../rtc_base:rtc_base_approved",
       "../rtc_base:rtc_base_tests_utils",
       "../system_wrappers",
+      "../test:perf_test",
       "../test:rtp_test_utils",
       "../test:test_common",
       "../test:test_renderer",
@@ -326,6 +327,7 @@
       "../system_wrappers:metrics_default",
       "../test:direct_transport",
       "../test:field_trial",
+      "../test:perf_test",
       "../test:rtp_test_utils",
       "../test:test_common",
       "../test:test_support",