Updated analysis in videoprocessor.

- Run analysis after all frames are processed. Before part of it was
done at bitrate change points;
- Analysis is done for whole stream as well as for each rate update
interval;
- Changed units from number of frames to time units for some metrics
and thresholds. E.g. 'num frames to hit tagret bitrate' is changed to
'time to reach target bitrate, sec';
- Changed data type of FrameStatistic::max_nalu_length (renamed to
max_nalu_size_bytes) from rtc::Optional to size_t. There it no need to
use such advanced data type in such low level data structure.

Bug: webrtc:8524
Change-Id: Ic9f6eab5b15ee12a80324b1f9c101de1bf3c702f
Reviewed-on: https://webrtc-review.googlesource.com/31901
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21653}

modules/video_coding/codecs/test/videoprocessor_unittest.cc

@@ -151,10 +151,6 @@ TEST_F(VideoProcessorTest, SetRates) {
                   kFramerateFps))
       .Times(1);
   video_processor_->SetRates(kBitrateKbps, kFramerateFps);
-  EXPECT_THAT(video_processor_->NumberDroppedFramesPerRateUpdate(),
-              ElementsAre(0));
-  EXPECT_THAT(video_processor_->NumberSpatialResizesPerRateUpdate(),
-              ElementsAre(0));
 
   const int kNewBitrateKbps = 456;
   const int kNewFramerateFps = 34;
@@ -164,10 +160,6 @@ TEST_F(VideoProcessorTest, SetRates) {
                   kNewFramerateFps))
       .Times(1);
   video_processor_->SetRates(kNewBitrateKbps, kNewFramerateFps);
-  EXPECT_THAT(video_processor_->NumberDroppedFramesPerRateUpdate(),
-              ElementsAre(0, 0));
-  EXPECT_THAT(video_processor_->NumberSpatialResizesPerRateUpdate(),
-              ElementsAre(0, 0));
 
   ExpectRelease();
 }