Reland "Clean up libjingle API dependencies."

This is a reland of 9185aca9ce1f66e983d9a5e797cab77a64cc46b0

> Original change's description:
> > > Clean up libjingle API dependencies.
> > > 
> > > This CL moves candidate.h into the public API, since it has
> > > been implicitly included before.
> > > 
> > > This is a straightforward way of solving the circular
> > > dependencies involving that file. For instance,
> > > libjingle_peerconnection_api includes candidate.h from
> > > jsepicecandidate.h, but _api can't depend on rtc_p2p, which
> > > depends on _api. In fact, _api can't depend on much at all
> > > since it's a very high level abstraction; instead, things
> > > should depend on it.
> > > 
> > > Furthermore, we have the case where deprecated headers
> > > include headers in internal modules. I just have to turn
> > > off include checking for those, but that's not a big deal.
> > > 
> > > This CL punts the problem of callfactoryinterface.h being
> > > implicitly included, and pulling in most of the call
> > > module with it. This should be addressed in a follow-up
> > > CL.
> Bug: webrtc:7504
> Change-Id: Icae0ba1a0488550e2871cc65e66d3661707aa5b6
> Reviewed-on: https://webrtc-review.googlesource.com/6460
> Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
> Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
> Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20156}

TBR=deadbeef@webrtc.org

Bug: webrtc:7504
Change-Id: Ic6598ac2af9355b60bbd289c86dc75e0ae9fed2e
Reviewed-on: https://webrtc-review.googlesource.com/6801
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20167}
diff --git a/rtc_base/network_constants.h b/rtc_base/network_constants.h
new file mode 100644
index 0000000..b4c8bea
--- /dev/null
+++ b/rtc_base/network_constants.h
@@ -0,0 +1,36 @@
+/*
+ *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_NETWORK_CONSTANTS_H_
+#define RTC_BASE_NETWORK_CONSTANTS_H_
+
+#include <stdint.h>
+
+namespace rtc {
+
+static const uint16_t kNetworkCostMax = 999;
+static const uint16_t kNetworkCostHigh = 900;
+static const uint16_t kNetworkCostUnknown = 50;
+static const uint16_t kNetworkCostLow = 10;
+static const uint16_t kNetworkCostMin = 0;
+
+enum AdapterType {
+  // This enum resembles the one in Chromium net::ConnectionType.
+  ADAPTER_TYPE_UNKNOWN = 0,
+  ADAPTER_TYPE_ETHERNET = 1 << 0,
+  ADAPTER_TYPE_WIFI = 1 << 1,
+  ADAPTER_TYPE_CELLULAR = 1 << 2,
+  ADAPTER_TYPE_VPN = 1 << 3,
+  ADAPTER_TYPE_LOOPBACK = 1 << 4
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_NETWORK_CONSTANTS_H_