Add mojo structs for ScopedFD and ScopedHandle.

mojo/common/test_common_custom_types.mojom

Index: mojo/common/test_common_custom_types.mojom
diff --git a/mojo/common/test_common_custom_types.mojom b/mojo/common/test_common_custom_types.mojom
index aa4cbc3a07a5c86dc92a9516c2e02c31caf7897a..45c861b733235ccd8782445008f858135709b133 100644
--- a/mojo/common/test_common_custom_types.mojom
+++ b/mojo/common/test_common_custom_types.mojom
@@ -6,6 +6,7 @@ module mojo.common.test;
 
 import "mojo/common/file.mojom";
 import "mojo/common/file_path.mojom";
+import "mojo/common/platform_handle.mojom";
 import "mojo/common/string16.mojom";
 import "mojo/common/time.mojom";
 import "mojo/common/unguessable_token.mojom";
@@ -47,3 +48,9 @@ interface TestFile {
   BounceFile(mojo.common.mojom.File? in)
       => (mojo.common.mojom.File? out);
 };
+
+interface TestPlatformHandle {
+  [Sync]
+  BouncePlatformHandle(mojo.common.mojom.PlatformHandle? in)
+      => (mojo.common.mojom.PlatformHandle? out);
+};