Add UMA metrics for ICE regathering reasons.

BUG=webrtc:6462
R=deadbeef@webrtc.org

Review URL: https://codereview.webrtc.org/2386783002 .

Cr-Commit-Position: refs/heads/master@{#14531}
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index fe8760f..d47cdf9 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -337,6 +337,7 @@
   rtc_test("rtc_unittests") {
     testonly = true
     sources = [
+      "api/fakemetricsobserver.cc",
       "base/array_view_unittest.cc",
       "base/atomicops_unittest.cc",
       "base/autodetectproxy_unittest.cc",