Separate out the platform-independent parts of Channel reading.

ipc/ipc_channel.h

Index: ipc/ipc_channel.h
diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index 983503b2e6b33a965790cc75d38c39e4009fea0c..770198a916bb8f23b822b59618481d5679db723e 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h
@@ -94,6 +94,17 @@ class IPC_EXPORT Channel : public Message::Sender {
 #endif
   };
 
+  // The Hello message is internal to the Channel class.  It is sent
+  // by the peer when the channel is connected.  The message contains
+  // just the process id (pid).  The message has a special routing_id
+  // (MSG_ROUTING_NONE) and type (HELLO_MESSAGE_TYPE).
+  enum {
+    HELLO_MESSAGE_TYPE = kuint16max  // Maximum value of message type (uint16),
+                                     // to avoid conflicting with normal
+                                     // message types, which are enumeration
+                                     // constants starting from 0.
+  };
+
   // The maximum message size in bytes. Attempting to receive a message of this
   // size or bigger results in a channel error.
   static const size_t kMaximumMessageSize = 128 * 1024 * 1024;
@@ -197,17 +208,6 @@ class IPC_EXPORT Channel : public Message::Sender {
   // PIMPL to which all channel calls are delegated.
   class ChannelImpl;
   ChannelImpl *channel_impl_;
-
-  // The Hello message is internal to the Channel class.  It is sent
-  // by the peer when the channel is connected.  The message contains
-  // just the process id (pid).  The message has a special routing_id
-  // (MSG_ROUTING_NONE) and type (HELLO_MESSAGE_TYPE).
-  enum {
-    HELLO_MESSAGE_TYPE = kuint16max  // Maximum value of message type (uint16),
-                                     // to avoid conflicting with normal
-                                     // message types, which are enumeration
-                                     // constants starting from 0.
-  };
 };
 
 }  // namespace IPC