Add SecureMessage definitions to CryptAuth.

components/proximity_auth/cryptauth/BUILD.gn

Index: components/proximity_auth/cryptauth/BUILD.gn
diff --git a/components/proximity_auth/cryptauth/BUILD.gn b/components/proximity_auth/cryptauth/BUILD.gn
index 59938c45dac0d863bc0c9922d35f7d2dbc8c8e3f..6407a74aeab97903a00aa2b6ff0494e88970779b 100644
--- a/components/proximity_auth/cryptauth/BUILD.gn
+++ b/components/proximity_auth/cryptauth/BUILD.gn
@@ -13,6 +13,8 @@ source_set("cryptauth") {
     "cryptauth_client.h",
     "cryptauth_enrollment_utils.cc",
     "cryptauth_enrollment_utils.h",
+    "secure_message_delegate.cc",
+    "secure_message_delegate.h",
   ]
 
   deps = [
@@ -26,6 +28,22 @@ source_set("cryptauth") {
   ]
 }
 
+source_set("test_support") {
+  sources = [
+    "fake_secure_message_delegate.cc",
+    "fake_secure_message_delegate.h",
+  ]
+
+  deps = [
+    ":cryptauth",
+    "//base",
+  ]
+
+  public_deps = [
+    "proto",
+  ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [