Adding back sigslot::repeater.

This time, with variadic template args, and unit tests!

NOPRESUBMIT=True

Bug: None
Change-Id: I2669cf5b24ab511eef8c01866748c1424b04abe1
Reviewed-on: https://webrtc-review.googlesource.com/4300
Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20171}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 8c3000e..8d93870 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -554,6 +554,7 @@
     "signalthread.h",
     "sigslot.cc",
     "sigslot.h",
+    "sigslotrepeater.h",
     "socket.h",
     "socketadapters.cc",
     "socketadapters.h",