Add a "Smart flushing" feature to NetEq.

Instead of flushing all packets, it makes sense to flush down to the target level instead. This CL also initiates a flush when the packet buffer is a multiple of the target level, instead of waiting until it is completely full.

Bug: webrtc:12201
Change-Id: I8775147624536824eb88752f6e8ffe57ec6199cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/193941
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Jakob Ivarsson <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32701}
diff --git a/api/neteq/neteq_controller.h b/api/neteq/neteq_controller.h
index 2c09c3e..4c49a0c 100644
--- a/api/neteq/neteq_controller.h
+++ b/api/neteq/neteq_controller.h
@@ -103,6 +103,7 @@
     uint16_t main_sequence_number;
     bool is_cng_or_dtmf;
     bool is_dtx;
+    bool buffer_flush;
   };
 
   virtual ~NetEqController() = default;