NetEq: Implement logging of Delayed Packet Outage Events

Measures the duration of each packet loss concealment (a.k.a. expand)
event that is not followed by a merge operation.

Having decoded and played packet m−1, the next expected packet is
m. If packet m arrives after some time of packet loss concealment, we
have a delayed packet outage event. However, if instead packet n>m
arrives, we have a lost packet outage event. In NetEq, the two outage
types results in different operations. Both types start with expand
operations to generate audio to play while the buffer is empty. When a
lost packet outage happens, the expand operation(s) are followed by
one merge operation. For delayed packet outages, merge is not done,
and the expand operations are immediately followed by normal
operations.

This change also includes unit tests for the new statistics.

BUG=webrtc:4915, chromium:488124
R=minyue@webrtc.org

Review URL: https://codereview.webrtc.org/1290113002 .

Cr-Commit-Position: refs/heads/master@{#9725}

webrtc/modules/audio_coding/neteq/neteq_impl.cc

@@ -1874,7 +1874,7 @@ void NetEqImpl::UpdatePlcComponents(int fs_hz, size_t channels) {
   // Delete objects and create new ones.
   expand_.reset(expand_factory_->Create(background_noise_.get(),
                                         sync_buffer_.get(), &random_vector_,
-                                        fs_hz, channels));
+                                        &stats_, fs_hz, channels));
   merge_.reset(new Merge(fs_hz, channels, expand_.get(), sync_buffer_.get()));
 }