Make the browser->metro_viewer IPC channel name a constant instead of hardcoding it in a few places…

win8/viewer/metro_viewer_process_host.cc

Index: win8/viewer/metro_viewer_process_host.cc
diff --git a/win8/viewer/metro_viewer_process_host.cc b/win8/viewer/metro_viewer_process_host.cc
index b04fa2199b566a12f05a4006ef5d4f0a36e2d6fb..512f14bff179f66e1c46e3978c08963c0467c0b3 100644
--- a/win8/viewer/metro_viewer_process_host.cc
+++ b/win8/viewer/metro_viewer_process_host.cc
@@ -19,6 +19,7 @@
 #include "ipc/ipc_message_macros.h"
 #include "ui/aura/remote_root_window_host_win.h"
 #include "ui/metro_viewer/metro_viewer_messages.h"
+#include "win8/viewer/metro_viewer_constants.h"
 
 namespace win8 {
 
@@ -32,11 +33,10 @@ void MetroViewerProcessHost::InternalMessageFilter::OnChannelConnected(
 }
 
 MetroViewerProcessHost::MetroViewerProcessHost(
-    const std::string& ipc_channel_name,
     base::SingleThreadTaskRunner* ipc_task_runner) {
 
   channel_.reset(new IPC::ChannelProxy(
-      ipc_channel_name.c_str(),
+      kMetroViewerIPCChannelName,
       IPC::Channel::MODE_NAMED_SERVER,
       this,
       ipc_task_runner));