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

ash/test/test_metro_viewer_process_host.h

Index: ash/test/test_metro_viewer_process_host.h
diff --git a/ash/test/test_metro_viewer_process_host.h b/ash/test/test_metro_viewer_process_host.h
index 63ca815c33d30a39ceabe6c4184ab693fcc413c4..e6d0a648cd93edee023faff698e28c9a0e793b2b 100644
--- a/ash/test/test_metro_viewer_process_host.h
+++ b/ash/test/test_metro_viewer_process_host.h
@@ -5,8 +5,7 @@
 #ifndef ASH_TEST_TEST_METRO_VIEWER_PROCESS_HOST_H_
 #define ASH_TEST_TEST_METRO_VIEWER_PROCESS_HOST_H_
 
-#include <string>
-
+#include "base/memory/scoped_ptr.h"
 #include "win8/viewer/metro_viewer_process_host.h"
 
 class AcceleratedSurface;
@@ -16,8 +15,7 @@ namespace test {
 
 class TestMetroViewerProcessHost : public win8::MetroViewerProcessHost {
  public:
-  TestMetroViewerProcessHost(const std::string& ipc_channel_name,
-                             base::SingleThreadTaskRunner* ipc_task_runner);
+  TestMetroViewerProcessHost(base::SingleThreadTaskRunner* ipc_task_runner);
   virtual ~TestMetroViewerProcessHost();
 
   bool closed_unexpectedly() { return closed_unexpectedly_; }
@@ -29,7 +27,7 @@ class TestMetroViewerProcessHost : public win8::MetroViewerProcessHost {
   virtual void OnOpenURL(const string16& url) OVERRIDE;
   virtual void OnHandleSearchRequest(const string16& search_string) OVERRIDE;
 
-  scoped_ptr<AcceleratedSurface> backing_surface;
+  scoped_ptr<AcceleratedSurface> backing_surface_;
 
   bool closed_unexpectedly_;