Move timestamp_extrapolator.h to rtc_base/time/

This moves it from an API directory (system_wrappers/include/) to a
non-API directory, which is exactly what we want for utilities like
this.

BUG=webrtc:8445

Change-Id: I51dfe8879c28c91bd1c667fc47b4892373671e0f
Reviewed-on: https://webrtc-review.googlesource.com/21540
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22569}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 32b05f7..50deea8 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -94,6 +94,7 @@
     "../rtc_base:rtc_task_queue",
     "../rtc_base:sequenced_task_checker",
     "../rtc_base:weak_ptr",
+    "../rtc_base/time:timestamp_extrapolator",
     "../system_wrappers",
   ]