Adds shared base class for data units.

This reduces code duplication and ensures common behavior
between the unit classes.

Bug: webrtc:9709
Change-Id: I9529ef10b3f538355f53250a2b67c6b4e250cce8
Reviewed-on: https://webrtc-review.googlesource.com/c/110901
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25690}

api/units/timestamp.cc

@@ -13,7 +13,7 @@
 #include "rtc_base/strings/string_builder.h"
 
 namespace webrtc {
-std::string ToString(const Timestamp& value) {
+std::string ToString(Timestamp value) {
   char buf[64];
   rtc::SimpleStringBuilder sb(buf);
   if (value.IsInfinite()) {