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/modules/video_coding/frame_buffer2_unittest.cc b/modules/video_coding/frame_buffer2_unittest.cc
index 2c21a86..13ad66c 100644
--- a/modules/video_coding/frame_buffer2_unittest.cc
+++ b/modules/video_coding/frame_buffer2_unittest.cc
@@ -194,7 +194,7 @@
                          });
     });
     if (max_wait_time == 0) {
-      time_controller_.AdvanceTime(TimeDelta::Millis(0));
+      time_controller_.AdvanceTime(TimeDelta::Zero());
     }
   }
 
@@ -304,7 +304,7 @@
   ExtractFrame(50);
   InsertFrame(pid, 1, ts, true, kFrameSize);
   InsertFrame(pid, 0, ts, false, kFrameSize);
-  time_controller_.AdvanceTime(TimeDelta::Millis(0));
+  time_controller_.AdvanceTime(TimeDelta::Zero());
 
   CheckFrame(0, pid, 0);
   CheckFrame(1, pid, 1);
diff --git a/modules/video_coding/timing/timing_unittest.cc b/modules/video_coding/timing/timing_unittest.cc
index c12de99..8633c0d 100644
--- a/modules/video_coding/timing/timing_unittest.cc
+++ b/modules/video_coding/timing/timing_unittest.cc
@@ -150,10 +150,10 @@
   timing.set_max_playout_delay(TimeDelta::Millis(20));
   EXPECT_FALSE(timing.RenderParameters().use_low_latency_rendering);
   // True if min==0, max > 0.
-  timing.set_min_playout_delay(TimeDelta::Millis(0));
+  timing.set_min_playout_delay(TimeDelta::Zero());
   EXPECT_TRUE(timing.RenderParameters().use_low_latency_rendering);
   // True if min==max==0.
-  timing.set_max_playout_delay(TimeDelta::Millis(0));
+  timing.set_max_playout_delay(TimeDelta::Zero());
   EXPECT_TRUE(timing.RenderParameters().use_low_latency_rendering);
   // True also for max playout delay==500 ms.
   timing.set_max_playout_delay(TimeDelta::Millis(500));