Include class relationship diagrams in network stack documentation.

net/docs/url_request.dot

+digraph URLRequestRoot {
+    subgraph cluster_legend {
+        label="Legend";
+
+        ## The following legend is an attempt to match UML notation,
+        ## except for template_class and Factory->object, which are
+        ## invented for this diagram.
+        BaseClass;
+        SubClass [label="Derived Class"];
+        Whole;
+        Part;
+        A;
+        B;
+        Interface [label="Interface / ABC", style=dashed];
+        template_class [shape=diamond]; # Link will name parameter(s)
+
+        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"];
+        // Often a "subgraph { rank=same; .. }" is used to wrap the
+        // below to make the generative relationship distinctive
+        // from the other class relationships.
+        Factory -> object [arrowhead=veevee];
+    };
+
+    ## URLRequest, URLRequestJob, and subclasses
+    URLRequestContext;
+    URLRequest;
+    URLRequestJob [style=dashed];
+    URLRequestJob_Others [label="...other job types..."];
+    URLRequestHttpJob;
+    Filter;
+
+    {URLRequestHttpJob, URLRequestJob_Others} -> URLRequestJob
+        [arrowhead="empty"];
+
+    URLRequestJob -> URLRequest [arrowhead="diamond"];
+    Filter -> URLRequestJob [arrowhead="diamond"];
+    Filter -> Filter [arrowhead="diamond", taillabel="0..1"];
+
+    subgraph {
+        rank=same;
+        URLRequestContext -> URLRequest [arrowhead=veevee];
+    }
+    subgraph {
+        rank=same;
+        URLRequestHttpJob -> Filter [arrowhead=veevee];
+    }
+
+
+    ## HttpTransaction, subclasses, and generative classes.
+    HttpTransactionFactory;
+    HttpCache;
+    HttpNetworkLayer;
+    HttpTransaction [style=dashed];
+    HttpCache_Transaction [label="HttpCache::Transaction"];
+    HttpNetworkTransaction;
+
+    { HttpNetworkTransaction, HttpCache_Transaction } -> HttpTransaction
+        [style=dashed, arrowhead="empty"];
+    { HttpNetworkLayer, HttpCache } -> HttpTransactionFactory
+        [arrowhead=empty, style=dashed];
+
+    HttpTransaction -> HttpCache_Transaction  [arrowhead=diamond];
+    HttpTransaction -> URLRequestHttpJob [arrowhead="diamond"]
+
+    subgraph {
+        rank=same;
+        HttpCache -> HttpCache_Transaction [arrowhead=veevee];
+    }
+    subgraph {
+        rank=same;
+        HttpTransactionFactory -> HttpTransaction [arrowhead=veevee];
+    }
+    subgraph {
+        rank=same;
+        HttpNetworkLayer -> HttpNetworkTransaction [arrowhead=veevee];
+    }
+
+    ## HttpStreamFactory and related.
+    HttpStreamFactoryImpl;
+    HttpStreamRequest [style=dashed];
+    HttpStream [style=dashed];
+    HttpStreamFactoryImpl_Request [label="HttpStreamFactoryImpl::Request"];
+    HttpStreamFactoryImpl_Job [label="HttpStreamFactoryImpl::Job"];
+    HttpStreamRequest_Delegate
+        [label="HttpStreamRequest::Delegate",style=dashed];
+    HttpBasicStream;
+    QuicHttpStream;
+    SpdyHttpStream;
+    HttpBasicState;
+
+    HttpNetworkTransaction -> HttpStreamRequest_Delegate
+        [style=dashed, arrowhead="empty"];
+    HttpStreamFactoryImpl_Request -> HttpStreamRequest
+        [style=dashed, arrowhead="empty"];
+    { HttpBasicStream, QuicHttpStream, SpdyHttpStream } -> HttpStream
+        [style=dashed, arrowhead="empty"];
+
+    HttpStreamRequest -> HttpNetworkTransaction [arrowhead="diamond"];
+    HttpStream -> HttpNetworkTransaction [arrowhead="diamond"];
+    HttpBasicState -> HttpBasicStream [arrowhead=diamond];
+    HttpStreamFactoryImpl_Job -> HttpStreamFactoryImpl_Request
+        [arrowhead="diamond",taillabel="1..*"];
+
+    HttpStreamRequest_Delegate -> HttpStreamFactoryImpl_Request
+        [arrowhead=odiamond];
+    HttpStreamFactoryImpl_Job -> HttpStreamFactoryImpl_Job
+        [arrowhead=odiamond, label="blocking_job_\nwaiting_job_"];
+
+    subgraph {
+        rank=same;
+        HttpStreamFactoryImpl -> HttpStreamRequest [arrowhead=veevee];
+    }
+    subgraph {
+        rank=same;
+        HttpStreamRequest -> HttpStream [arrowhead=veevee];
+    }
+
+    ## ClientSocketHandle and socket pools.
+    ClientSocketPool [style=dashed];
+    TransportClientSocketPool;
+    SSLClientSocketPool;
+    ClientSocketPool_Others [label="...others..."];
+    ClientSocketPoolBase [label="ClientSocketPoolBase", shape=diamond];
+    ClientSocketPoolBaseHelper;
+
+    ConnectJobFactory;
+    ConnectJob [style=dashed];
+    TransportConnectJob;
+    SSLConnectJob;
+    ConnectJob_Others [label="...other connect job types..."];
+    ConnectJob_Delegate [label="ConnectJob::Delegate",style=dashed];
+
+    StreamSocket [style=dashed];
+    TCPClientSocket;
+    StreamSocket_Others [label="...other socket types..."];
+
+    TransportConnectJobHelper;
+    SingleRequestHostResolver;
+
+    { SSLClientSocketPool, TransportClientSocketPool,
+      ClientSocketPool_Others} -> ClientSocketPool
+        [style=dashed, arrowhead=empty];
+    ClientSocketPoolBaseHelper -> ConnectJob_Delegate
+        [arrowhead=empty, style=dashed];
+    StreamSocket -> Socket [arrowhead=empty, style=dashed];
+    { TCPClientSocket, StreamSocket_Others } -> StreamSocket
+        [arrowhead=empty, style=dashed];
+    {SSLConnectJob, TransportConnectJob, ConnectJob_Others} -> ConnectJob
+        [style=dashed, arrowhead=empty];
+
+    ClientSocketHandle -> HttpStreamFactoryImpl_Job [arrowhead="diamond"];
+    ClientSocketHandle -> HttpBasicState [arrowhead="diamond"];
+    ClientSocketPoolBaseHelper -> ClientSocketPoolBase [arrowhead=diamond];
+    ClientSocketPoolBase -> TransportClientSocketPool
+        [arrowhead=diamond, label=TransportSocketParams];
+    ClientSocketPoolBase -> SSLClientSocketPool
+        [arrowhead=diamond, label=SSLSocketParams];
+
+    StreamSocket -> ClientSocketHandle [arrowhead=diamond];
+    ConnectJobFactory -> ClientSocketPoolBase [arrowhead=diamond];
+    StreamSocket -> ConnectJob [arrowhead=diamond];
+    SingleRequestHostResolver -> TransportConnectJobHelper
+        [arrowhead=diamond];
+    TransportConnectJobHelper -> TransportConnectJob [arrowhead=diamond];
+
+    ClientSocketPool -> ClientSocketHandle  [arrowhead=odiamond];
+    ConnectJob_Delegate -> ConnectJob  [arrowhead=odiamond];
+
+    subgraph {
+        rank=same;
+        ConnectJobFactory -> ConnectJob [arrowhead=veevee];
+    }
+    subgraph {
+        rank=same;
+        HttpStreamFactoryImpl_Job -> ClientSocketHandle [arrowhead=veevee];
+    }
+    subgraph {
+        rank=same;
+        TransportConnectJob -> StreamSocket [arrowhead=veevee];
+    }
+}