Store video_quality_loopback_test perf results in Chart JSON format.

Adds a flag to store the perf results in a JSON file using the Chart
JSON format [1].

[1] https://github.com/catapult-project/catapult/blob/master/dashboard/docs/data-format.md

TBR=phoglund@webrtc.org

Bug: chromium:755660
Change-Id: I6a896654a4a558df217ddefa4e8a52a487cdbebd
Reviewed-on: https://webrtc-review.googlesource.com/43180
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21809}

rtc_tools/compare_videos.py

@@ -62,6 +62,8 @@ def _ParseArgs():
                     help='Width of the YUV file\'s frames. Default: %default')
   parser.add_option('--yuv_frame_height', type='int', default=480,
                     help='Height of the YUV file\'s frames. Default: %default')
+  parser.add_option('--chartjson_result_file', type='str', default=None,
+                    help='Where to store perf results in chartjson format.')
   options, _ = parser.parse_args()
 
   if options.stats_file:
@@ -161,6 +163,8 @@ def main():
     '--width=%d' % options.yuv_frame_width,
     '--height=%d' % options.yuv_frame_height,
   ]
+  if options.chartjson_result_file:
+    cmd.append('--chartjson_result_file=%s' % options.chartjson_result_file)
   frame_analyzer = subprocess.Popen(cmd, stdin=_DevNull(),
                                     stdout=sys.stdout, stderr=sys.stderr)
   frame_analyzer.wait()