Make NQE classes private to //net

Classes outside of //net should use
Network Quality Tracker instead of
Network Quality Estimator (NQE).

Change-Id: Iaf9d940c862f322f14eacb20391354e5cc7cb9cd
Bug: 878844
Reviewed-on: https://chromium-review.googlesource.com/c/1305203Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603680}

net/nqe/effective_connection_type_observer.h

@@ -13,7 +13,7 @@
 namespace net {
 
 // Observes changes in effective connection type.
-class NET_EXPORT EffectiveConnectionTypeObserver {
+class NET_EXPORT_PRIVATE EffectiveConnectionTypeObserver {
  public:
   // Notifies the observer of a change in the effective connection type.
   // NetworkQualityEstimator computes the effective connection type once in

net/nqe/network_quality_estimator.h

@@ -63,7 +63,7 @@ class URLRequest;
 // A single instance of NQE can be attached to multiple URLRequestContexts,
 // thereby increasing the single NQE instance's accuracy by providing more
 // observed traffic characteristics.
-class NET_EXPORT NetworkQualityEstimator
+class NET_EXPORT_PRIVATE NetworkQualityEstimator
     : public NetworkChangeNotifier::ConnectionTypeObserver {
  public:
   // Observes measurements of round trip time.

net/nqe/network_quality_store.h

@@ -27,7 +27,7 @@ namespace internal {
 class NET_EXPORT_PRIVATE NetworkQualityStore {
  public:
   // Observes changes in the cached network qualities.
-  class NET_EXPORT NetworkQualitiesCacheObserver {
+  class NET_EXPORT_PRIVATE NetworkQualitiesCacheObserver {
    public:
     // Notifies the observer of a change in the cached network quality. The
     // observer must register and unregister itself on the IO thread. All the

net/nqe/rtt_throughput_estimates_observer.h

@@ -15,7 +15,7 @@
 namespace net {
 
 // Observes changes in the network quality.
-class NET_EXPORT RTTAndThroughputEstimatesObserver {
+class NET_EXPORT_PRIVATE RTTAndThroughputEstimatesObserver {
  public:
   // Notifies the observer when estimated HTTP RTT, estimated transport RTT or
   // estimated downstream throughput is computed. NetworkQualityEstimator