Adds associated interface support to IPC::Channel

ipc/BUILD.gn

Index: ipc/BUILD.gn
diff --git a/ipc/BUILD.gn b/ipc/BUILD.gn
index 34cab687770f70256f4f100a1dad636542e491ff..8dba71da667800c90c63c64e67214ec1a0bee1fb 100644
--- a/ipc/BUILD.gn
+++ b/ipc/BUILD.gn
@@ -167,6 +167,13 @@ mojom("mojom") {
   ]
 }
 
+mojom("test_interfaces") {
+  testonly = true
+  sources = [
+    "ipc_test.mojom",
+  ]
+}
+
 # This is provided as a separate target so other targets can provide param
 # traits implementations without necessarily linking to all of IPC.
 source_set("param_traits") {
@@ -235,6 +242,7 @@ if (!is_ios) {
       ":ipc",
       ":mojom",
       ":run_all_unittests",
+      ":test_interfaces",
       ":test_support",
       "//base",
       "//base:i18n",