Efficient Metric Recorder

Computing all metrics using constant extra memory.
PlotHistogram methods are executed in constant time.
-- Previously throughput and delay were using O(num_packets) extra memory and their associated PlotHistograms took linear time complexity.

Added MetricRecorder unittests

R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1257683006 .

Cr-Commit-Position: refs/heads/master@{#9658}
diff --git a/webrtc/modules/modules.gyp b/webrtc/modules/modules.gyp
index 4c397f2..a769aa9 100644
--- a/webrtc/modules/modules.gyp
+++ b/webrtc/modules/modules.gyp
@@ -224,6 +224,7 @@
             'remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.h',
             'remote_bitrate_estimator/test/bwe_test_framework_unittest.cc',
             'remote_bitrate_estimator/test/bwe_unittest.cc',
+            'remote_bitrate_estimator/test/metric_recorder_unittest.cc',
             'remote_bitrate_estimator/test/estimators/nada_unittest.cc',
             'rtp_rtcp/source/mock/mock_rtp_payload_strategy.h',
             'rtp_rtcp/source/byte_io_unittest.cc',