Fix about:webrtc-internals "Stats Tables"

This is a fixup CL of [1] spotted and report by hbos@chromium.org
on [2].

[1] https://crrev.com/c/1842414 (Onion soup RTCPeerConnectionHandler and its direct dependences)
[2] https://crrev.com/c/1842414/14#message-daf2b117b98895c3692a635ac873e249e6f905c1

Basically the original CL was supposed to replace the code
that used to bound PeerConnectionTracker to its associated
mojom::Receiver instance in [3] by the corresponding code in
Blink.
However, the replacement code got accidentally missing while rebasing
the CL over and over.

[3] https://crrev.com/c/1842414/14/content/renderer/render_thread_impl.cc#b777

This CL amends the original CL with the proper code in
blink/renderer/modules/modules_initializer.cc@RegisterInterfaces().

TEST=<out>/browser_tests --gtest_filter=WebRtcInternalsPerfBrowserTest.MANUAL_RunsAudioVideoCall60SecsAndLogsInternalMetricsVp8 \
--run-manual --ui-test-action-max-timeout=100000 --test-launcher-timeout=100001

Manual test= Open Chromium load about:webrtc-internals and [4].
It is expected that WebRTC statistics appear.

[4] https://codepen.io/anon/pen/JQLEqR?editors=1010

R=haraken@chromium.org
TBR=guidou@chromium.org, hbos@chromium.org

BUG=787254

Change-Id: I88f26b8cbcc09b707f6db5c9e7ba40bdbd8e82e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893293Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711647}

third_party/blink/renderer/modules/modules_initializer.cc

@@ -62,6 +62,7 @@
 #include "third_party/blink/renderer/modules/media_controls/media_controls_impl.h"
 #include "third_party/blink/renderer/modules/mediastream/user_media_client.h"
 #include "third_party/blink/renderer/modules/mediastream/user_media_controller.h"
+#include "third_party/blink/renderer/modules/peerconnection/peer_connection_tracker.h"
 #include "third_party/blink/renderer/modules/picture_in_picture/picture_in_picture_controller_impl.h"
 #include "third_party/blink/renderer/modules/presentation/presentation_controller.h"
 #include "third_party/blink/renderer/modules/presentation/presentation_receiver.h"
@@ -316,6 +317,11 @@ void ModulesInitializer::RegisterInterfaces(
   registry.AddInterface(
       ConvertToBaseCallback(CrossThreadBindRepeating(&WebDatabaseImpl::Create)),
       Platform::Current()->GetIOTaskRunner());
+  registry.AddInterface(
+      ConvertToBaseCallback(CrossThreadBindRepeating(
+          &PeerConnectionTracker::Bind,
+          WTF::CrossThreadUnretained(PeerConnectionTracker::GetInstance()))),
+      Thread::MainThread()->GetTaskRunner());
 }
 
 }  // namespace blink