Revert "Unregister RTCPeerConnectionHandler upon native PC's Close()"

This reverts commit d138e38a.

Reason for revert: assessing if cause for https://bugs.chromium.org/p/chromium/issues/detail?id=783010

Original change's description:
> Unregister RTCPeerConnectionHandler upon native PC's Close()
> 
> The existing code only unregisters the handler it is destructed. 
> However, garbage collection might not destroy the object for a while.
> 
> Bug: 780855
> Change-Id: I68d2c1b49cc5042d53464878977400bfd3b75c81
> Reviewed-on: https://chromium-review.googlesource.com/754838
> Commit-Queue: Elad Alon <eladalon@chromium.org>
> Reviewed-by: Henrik Boström <hbos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#514466}

TBR=hbos@chromium.org,guidou@chromium.org,terelius@chromium.org,eladalon@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 780855
Change-Id: I567bfb7f5aa7f2658b1dbee06a3fe5b66f3dec4b
Reviewed-on: https://chromium-review.googlesource.com/765147Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Benjamin Joyce <bjoyce@google.com>
Cr-Commit-Position: refs/heads/master@{#515870}

content/renderer/media/rtc_peer_connection_handler.cc

@@ -1199,6 +1199,10 @@ RTCPeerConnectionHandler::~RTCPeerConnectionHandler() {
 
   Stop();
 
+  GetPeerConnectionHandlers()->erase(this);
+  if (peer_connection_tracker_)
+    peer_connection_tracker_->UnregisterPeerConnection(this);
+
   UMA_HISTOGRAM_COUNTS_10000(
       "WebRTC.NumDataChannelsPerPeerConnection", num_data_channels_created_);
 }
@@ -1924,11 +1928,6 @@ void RTCPeerConnectionHandler::Stop() {
 
   native_peer_connection_->Close();
 
-  GetPeerConnectionHandlers()->erase(this);
-  if (peer_connection_tracker_)
-    peer_connection_tracker_->UnregisterPeerConnection(this);
-  peer_connection_tracker_ = nullptr;
-
   // This object may no longer forward call backs to blink.
   is_closed_ = true;
 }