Add ScopedAllowIO in WebrtcTransport when shutting down WebRTC session.

PeerConnection creates new threads internally and shuts them down when
connection is closed. This is a blocking operation, so Debug builds were
DCHECK'ing every time connection is terminated. Added ScopedAllowIO to
work around this issue.

BUG=660081

Review-Url: https://codereview.chromium.org/2567533002
Cr-Commit-Position: refs/heads/master@{#437711}

PRESUBMIT.py

@@ -216,6 +216,7 @@ _BANNED_CPP_FUNCTIONS = (
         r"^net[\\\/]test[\\\/]spawned_test_server[\\\/]local_test_server\.cc$",
         r"^net[\\\/]test[\\\/]test_data_directory\.cc$",
         r"^net[\\\/]url_request[\\\/]test_url_fetcher_factory\.cc$",
+        r"^remoting[\\\/]protocol[\\\/]webrtc_transport\.cc$",
         r"^ui[\\\/]base[\\\/]material_design[\\\/]"
             "material_design_controller\.cc$",
         r"^ui[\\\/]gl[\\\/]init[\\\/]gl_initializer_mac\.cc$",

remoting/protocol/webrtc_transport.cc

@@ -18,6 +18,7 @@
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "base/task_runner_util.h"
+#include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "jingle/glue/thread_wrapper.h"
 #include "remoting/protocol/authenticator.h"
@@ -194,7 +195,13 @@ class WebrtcTransport::PeerConnectionWrapper
     peer_connection_ = peer_connection_factory_->CreatePeerConnection(
         rtc_config, &constraints, std::move(port_allocator), nullptr, this);
   }
-  virtual ~PeerConnectionWrapper() { peer_connection_->Close(); }
+  virtual ~PeerConnectionWrapper() {
+    // PeerConnection creates threads internally, which are stopped when the
+    // connection is closed. Thread.Stop() is a blocking operation.
+    // See crbug.com/660081.
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
+    peer_connection_->Close();
+  }
 
   WebrtcAudioModule* audio_module() {
     return audio_module_.get();