Add the target for overrides of metrics utilities in jingle_unittests.

The target rtc_p2p from WebRTC, transitively depended on by
jingle_unittests via the target webrtc_overrides, will soon depend on
the implementation of RTC_HISTOGRAM_* macros in WebRTC.

Bug: webrtc:9409
Change-Id: I1f2bf0776d2f7c18c9585fb15d7251f8128ce175
Reviewed-on: https://chromium-review.googlesource.com/1134183
Commit-Queue: Qingsi Wang <qingsi@google.com>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574376}

jingle/BUILD.gn

@@ -174,6 +174,7 @@ test("jingle_unittests") {
   public_deps = [
     "//third_party/libjingle_xmpp",
     "//third_party/webrtc_overrides",
+    "//third_party/webrtc_overrides:init_webrtc",
   ]
   deps = [
     ":jingle_glue",