Fix missing GN dependencies.

build/json_schema_api.gni

Index: build/json_schema_api.gni
diff --git a/build/json_schema_api.gni b/build/json_schema_api.gni
index aa6365b948ee505484650961634ee9aee2c35055..e1c2d33409d52ad4d92f6e233e3ce31f84789574 100644
--- a/build/json_schema_api.gni
+++ b/build/json_schema_api.gni
@@ -154,7 +154,9 @@ template("json_schema_api") {
 
     assert(defined(invoker.impl_dir),
            "\"impl_dir\" must be defined for the $target_name template.")
-    impl_dir = invoker.impl_dir
+
+    # Child directory inside the generated file tree.
+    gen_child_dir = rebase_path(invoker.impl_dir, "//")
 
     bundle_generator_registration_name =
         target_name + "_bundle_generator_registration"
@@ -162,15 +164,15 @@ template("json_schema_api") {
       script = compiler_script
       inputs = compiler_sources + invoker.sources + uncompiled_sources
       outputs = [
-        "$root_gen_dir/$impl_dir/generated_api_registration.cc",
-        "$root_gen_dir/$impl_dir/generated_api_registration.h",
+        "$root_gen_dir/$gen_child_dir/generated_api_registration.cc",
+        "$root_gen_dir/$gen_child_dir/generated_api_registration.h",
       ]
       args = [
                "--root=" + rebase_path("//", root_build_dir),
                "--destdir=" + rebase_path(root_gen_dir, root_build_dir),
                "--namespace=$root_namespace",
                "--generator=cpp-bundle-registration",
-               "--impl-dir=" + rebase_path(impl_dir, "//"),
+               "--impl-dir=$gen_child_dir",
                "--include-rules=$schema_include_rules",
              ] + rebase_path(invoker.sources, root_build_dir) +
              rebase_path(uncompiled_sources, root_build_dir)