Support generic synchronization against an IPC::Channel

ipc/BUILD.gn

Index: ipc/BUILD.gn
diff --git a/ipc/BUILD.gn b/ipc/BUILD.gn
index dc3694992946cf325a4c8ca4737224efb556a225..d2464b9a15113da985f3eac67b861221cbb22600 100644
--- a/ipc/BUILD.gn
+++ b/ipc/BUILD.gn
@@ -138,6 +138,7 @@ component("ipc") {
 
   public_deps = [
     ":param_traits",
+    "//mojo/common:event",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/system",
   ]
@@ -161,6 +162,10 @@ mojom("mojom") {
   sources = [
     "ipc.mojom",
   ]
+
+  public_deps = [
+    "//mojo/common:event",
+  ]
 }
 
 mojom("test_interfaces") {
@@ -168,6 +173,10 @@ mojom("test_interfaces") {
   sources = [
     "ipc_test.mojom",
   ]
+
+  public_deps = [
+    "//mojo/common:event",
+  ]
 }
 
 # This is provided as a separate target so other targets can provide param