Reland "stats: migrate to Timestamp"

This is a reland of commit 2235776597e2f47ec353ac911428eb9a54d64a10

Original change's description:
> stats: migrate to Timestamp
>
> BUG=webrtc:13756
>
> Change-Id: I04ba57f9c2ca5a974a406814023911b4eb2d6d38
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/273942
> Commit-Queue: Philipp Hancke <phancke@microsoft.com>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#38365}

Bug: webrtc:13756
Change-Id: Ib8dc208197ae5e90f67114e7b043a73ee35421ea
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/279080
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38380}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 388c289..6c9414e 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -706,6 +706,7 @@
     "../rtc_base:checks",
     "../rtc_base:refcount",
     "../rtc_base/system:rtc_export",
+    "units:timestamp",
   ]
 }