Move ScopedIPCSupport into its own build rule.

ipc/mojo/scoped_ipc_support.h

Index: ipc/mojo/scoped_ipc_support.h
diff --git a/ipc/mojo/scoped_ipc_support.h b/ipc/mojo/scoped_ipc_support.h
index 29c50bf4a48ba48905701feede928a3d1fc045a7..1ef26fc97ca30f0749a181782f32a8b7c234e7b4 100644
--- a/ipc/mojo/scoped_ipc_support.h
+++ b/ipc/mojo/scoped_ipc_support.h
@@ -8,7 +8,6 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/task_runner.h"
-#include "ipc/ipc_export.h"
 
 namespace IPC {
 
@@ -18,7 +17,7 @@ namespace IPC {
 // This should be instantiated once per process and retained as long as Mojo IPC
 // is needed. The TaskRunner passed to the constructor should outlive this
 // object.
-class IPC_MOJO_EXPORT ScopedIPCSupport {
+class ScopedIPCSupport {
  public:
   ScopedIPCSupport(scoped_refptr<base::TaskRunner> io_thread_task_runner);
   ~ScopedIPCSupport();