clean up misc TimeDelta use

follow-up from https://webrtc-review.googlesource.com/c/src/+/262810

* replace Time::Millis(0) and TimeDelta::Millis(0) with ::Zero()
* drop unnecessary webrtc namespace from some TimeDeltas
* make TimeDelta do the unit conversion for stats

BUG=webrtc:13756

Change-Id: Ic60625ae0fc7959a47a6be9f5051851feaf76373
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/265875
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37664}
diff --git a/video/receive_statistics_proxy2.cc b/video/receive_statistics_proxy2.cc
index 5f1f635..3448ab5 100644
--- a/video/receive_statistics_proxy2.cc
+++ b/video/receive_statistics_proxy2.cc
@@ -769,10 +769,10 @@
                                             absl::optional<uint8_t> qp,
                                             TimeDelta decode_time,
                                             VideoContentType content_type) {
-  webrtc::TimeDelta processing_delay = webrtc::TimeDelta::Millis(0);
+  TimeDelta processing_delay = TimeDelta::Zero();
   webrtc::Timestamp current_time = clock_->CurrentTime();
   // TODO(bugs.webrtc.org/13984): some tests do not fill packet_infos().
-  webrtc::TimeDelta assembly_time = webrtc::TimeDelta::Millis(0);
+  TimeDelta assembly_time = TimeDelta::Zero();
   if (frame.packet_infos().size() > 0) {
     const auto [first_packet, last_packet] = std::minmax_element(
         frame.packet_infos().cbegin(), frame.packet_infos().cend(),
@@ -803,8 +803,8 @@
     const VideoFrameMetaData& frame_meta,
     absl::optional<uint8_t> qp,
     TimeDelta decode_time,
-    webrtc::TimeDelta processing_delay,
-    webrtc::TimeDelta assembly_time,
+    TimeDelta processing_delay,
+    TimeDelta assembly_time,
     VideoContentType content_type) {
   RTC_DCHECK_RUN_ON(&main_thread_);