Include class relationship diagrams in network stack documentation.

net/docs/url_request.dot

+digraph URLRequestRoot {
+        subgraph cluster_legend {
+            label="Legend";
+
+            ## UML
+            SubClass -> BaseClass [arrowhead="empty"];
+            SubClass -> Interface [arrowhead="empty", style=dashed];
+            Part -> Whole [arrowhead="diamond", label="ownership"];
+            Part -> Whole [arrowhead="odiamond", label="pointer"];
+            A -> B [arrowhead="none", headlabel="?..?", taillabel="?..?",
+                    label="association"];
+            Interface [label="Interface / ABC", style=dashed];
+            template_class [shape=diamond]; # Link will name parameter(s)
+
+
+            Factory -> object [arrowhead=veevee];
+            SubClass [label="Derived Class"];
+        };
+
+        URLRequest;  // Non-threadsafe, SupportsUserData
+        URLRequestContext;
+        URLRequestJob [style=dashed];  // RC, base::PowerObserver
+                        // Many subclasses
+        HttpCache_Transaction [label="HttpCache::Transaction"];
+        HttpStreamFactoryImpl_Request [label="HttpStreamFactoryImpl::Request"];
+        HttpStreamFactoryImpl_Job [label="HttpStreamFactoryImpl::Job"];
+        HttpStreamRequest_Delegate [label="HttpStreamRequest::Delegate",style=dashed];
+        Filter;
+
+        URLRequestContext -> URLRequest [arrowhead=veevee];
+        Filter -> URLRequestJob [arrowhead="diamond"];
+        Filter -> Filter [arrowhead="diamond", taillabel="0..1"];
+        URLRequestHttpJob -> Filter [arrowhead=veevee];
+
+        URLRequestJob -> URLRequest [arrowhead="diamond"];
+
+        URLRequestHttpJob -> URLRequestJob [arrowhead="empty"];
+                                // Handles both http & https
+
+        URLRequestJobOthers [label="...others..."];
+        URLRequestJobOthers -> URLRequestJob [arrowhead="empty"];
+
+        // Indirects to network delegate.
+
+        HttpCache;
+        HttpNetworkLayer;
+        HttpTransactionFactory;
+        HttpTransaction [style=dashed];
+        HttpTransaction_Others [label="...others..."];
+        HttpTransaction -> URLRequestHttpJob [arrowhead="diamond"]
+
+        HttpNetworkLayer -> HttpTransactionFactory [arrowhead=empty, style=dashed];
+
+        HttpStreamFactoryImpl_Job -> HttpStreamFactoryImpl_Request [arrowhead="diamond",taillabel="1..*"];
+
+        ClientSocketHandle -> {HttpStreamFactoryImpl_Job,HttpBasicState} [arrowhead="diamond"];
+        ClientSocketPool [style=dashed];
+        ClientSocketPool -> ClientSocketHandle  [arrowhead=odiamond];
+
+        {SSLClientSocketPool, TransportClientSocketPool, ClientSocketPool_Others} -> ClientSocketPool [style=dashed, arrowhead=empty];
+        ClientSocketPool_Others [label="...others..."];
+
+        ClientSocketPoolBase -> TransportClientSocketPool [arrowhead=diamond, label=TransportSocketParams];
+        ClientSocketPoolBase [label="ClientSocketPoolBase", shape=diamond];
+
+        ClientSocketPoolBaseHelper -> ConnectJob_Delegate [arrowhead=empty, style=dashed];
+        ConnectJob [style=dashed];
+        ConnectJob_Delegate [label="ConnectJob::Delegate",style=dashed];
+
+        StreamSocket [style=dashed];
+        StreamSocket -> Socket [arrowhead=empty, style=dashed];
+        TCPClientSocket -> StreamSocket [arrowhead=empty, style=dashed];
+        StreamSocket_others -> StreamSocket [arrowhead=empty, style=dashed];
+        StreamSocket_others [label="...others..."];
+        StreamSocket -> ClientSocketHandle [arrowhead=diamond];
+        ConnectJobFactory -> ClientSocketPoolBase [arrowhead=diamond];
+        subgraph { rank=same; ConnectJobFactory -> ConnectJob [arrowhead=veevee];}
+        StreamSocket -> ConnectJob [arrowhead=diamond];
+
+        TransportConnectJob;
+        TransportConnectJobHelper;
+        SingleRequestHostResolver;
+
+        SingleRequestHostResolver -> TransportConnectJobHelper ->
+            TransportConnectJob [arrowhead=diamond];
+
+        {SSLConnectJob, TransportConnectJob, ConnectJob_Others} -> ConnectJob [style=dashed, arrowhead=empty];
+        ConnectJob_Others [label="...others..."];
+
+        ConnectJob_Delegate -> ConnectJob  [arrowhead=odiamond];
+        ClientSocketPoolBaseHelper -> ClientSocketPoolBase [arrowhead=diamond];
+
+        subgraph {
+          rank=same;
+          URLRequestJobManager -> URLRequestJob [arrowhead=veevee];
+        }
+
+        TransportClientSocketPool -> ClientSocketHandle [arrowhead=veevee];
+
+        HttpTransactionFactory [style=dashed];
+        HttpCache -> HttpTransactionFactory [arrowhead=empty, style=dashed];
+        HttpTransactionFactory -> HttpTransaction [arrowhead=veevee];
+        HttpNetworkLayer -> HttpNetworkTransaction [arrowhead=veevee];
+        HttpCache -> HttpCache_Transaction [arrowhead=veevee];
+
+         HttpStreamRequest_Delegate -> HttpStreamFactoryImpl_Request [arrowhead=odiamond];
+        subgraph { rank=same; HttpStreamFactoryImpl -> HttpStreamRequest -> HttpStream [arrowhead=veevee];}
+
+        HttpTransaction_Others -> HttpTransaction [style=dashed, arrowhead="empty"]; 
+        HttpCache_Transaction -> HttpTransaction [style=dashed, arrowhead="empty"];
+        HttpTransaction -> HttpCache_Transaction  [arrowhead=diamond];
+        HttpNetworkTransaction -> HttpTransaction [style=dashed, arrowhead="empty"];
+
+        HttpStreamRequest [style=dashed];
+        HttpStreamRequest -> HttpNetworkTransaction [arrowhead="diamond"];
+        HttpStream -> HttpNetworkTransaction [arrowhead="diamond"];
+        HttpNetworkTransaction -> HttpStreamRequest_Delegate [style=dashed, arrowhead="empty"];
+        HttpStreamFactoryImpl_Request -> HttpStreamRequest [style=dashed, arrowhead="empty"];
+        HttpStream [style=dashed];
+        HttpBasicState -> HttpBasicStream [arrowhead=diamond];
+        HttpBasicStream -> HttpStream [style=dashed, arrowhead="empty"];
+        QuicHttpStream -> HttpStream [style=dashed, arrowhead="empty"];
+        SpdyHttpStream -> HttpStream [style=dashed, arrowhead="empty"];
+}