Add "implemented_in" key in IDL schema compiler

tools/json_schema_compiler/test/idl_namespace_specific_implement.idl

Index: tools/json_schema_compiler/test/idl_namespace_specific_implement.idl
diff --git a/tools/json_schema_compiler/test/idl_namespace_all_platforms.idl b/tools/json_schema_compiler/test/idl_namespace_specific_implement.idl
similarity index 50%
copy from tools/json_schema_compiler/test/idl_namespace_all_platforms.idl
copy to tools/json_schema_compiler/test/idl_namespace_specific_implement.idl
index 7ffd84a60046509eac710265fa5ca4a166d80bc3..99b65ac50a53fcf5cd73cd38ba01a0be1515b598 100644
--- a/tools/json_schema_compiler/test/idl_namespace_all_platforms.idl
+++ b/tools/json_schema_compiler/test/idl_namespace_specific_implement.idl
@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Tests a variety of basic API definition features.
+// Tests the case of a namespace with specific implemented file.
 
-[platforms=("chromeos", "chromeos_touch", "linux", "mac", "win")]
-namespace idl_namespace_all_platforms {
+[implemented_in="idl_namespace_specific_implement.idl"]
+namespace idl_namespace_specific_implement {
 
 };