Move ScopedIPCSupport into its own build rule.

ipc/mojo/BUILD.gn

Index: ipc/mojo/BUILD.gn
diff --git a/ipc/mojo/BUILD.gn b/ipc/mojo/BUILD.gn
index 22f063aaf836b724e4a0d3c444586144ce1e3c4e..fddd1a1a122389df73e26a7e7d7db31a2b8bd129 100644
--- a/ipc/mojo/BUILD.gn
+++ b/ipc/mojo/BUILD.gn
@@ -25,8 +25,6 @@ component("mojo") {
     "ipc_mojo_message_helper.h",
     "ipc_mojo_param_traits.cc",
     "ipc_mojo_param_traits.h",
-    "scoped_ipc_support.cc",
-    "scoped_ipc_support.h",
   ]
 
   defines = [ "IPC_MOJO_IMPLEMENTATION" ]
@@ -36,11 +34,23 @@ component("mojo") {
     "//base",
     "//base/third_party/dynamic_annotations",
     "//ipc",
-    "//mojo/edk/system",
+    "//mojo/public/c/system",
     "//mojo/public/cpp/bindings",
   ]
 }
 
+source_set("ipc_support") {
+  sources = [
+    "scoped_ipc_support.cc",
+    "scoped_ipc_support.h",
+  ]
+
+  deps = [
+    "//base",
+    "//mojo/edk/system",
+  ]
+}
+
 test("ipc_mojo_unittests") {
   sources = [
     "ipc_channel_mojo_unittest.cc",