fuchsia: Don't define fidl js (or cpp) compile action unless necessary

Bug: 898088
Change-Id: Iaf7c0013ff0c7fa64f008023619962526bc41b81
Reviewed-on: https://chromium-review.googlesource.com/c/1298223Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602394}

build/config/fuchsia/fidl_library.gni

@@ -45,6 +45,17 @@ template("fidl_library") {
     languages = [ "cpp" ]
   }
 
+  _define_cpp_action = false
+  _define_js_action = false
+
+  foreach(language, languages) {
+    if (language == "cpp") {
+      _define_cpp_action = true
+    } else if (language == "js") {
+      _define_js_action = true
+    }
+  }
+
   _response_file = "$target_gen_dir/$target_name.rsp"
   _json_representation = "$target_gen_dir/${_library_name}.fidl.json"
   _output_gen_dir = "$target_gen_dir/fidl"
@@ -139,6 +150,7 @@ template("fidl_library") {
     ]
   }
 
+  if (_define_cpp_action) {
     action("${target_name}_cpp_gen") {
       visibility = [ ":${invoker.target_name}" ]
       forward_variables_from(invoker, [ "testonly" ])
@@ -172,7 +184,9 @@ template("fidl_library") {
         rebase_path("${_output_base}"),
       ]
     }
+  }
 
+  if (_define_js_action) {
     _output_js_path = "$_output_gen_dir/${_library_path}/js/fidl.js"
     action("${target_name}_js_gen") {
       visibility = [ ":${invoker.target_name}" ]
@@ -206,6 +220,7 @@ template("fidl_library") {
         data += [ rebase_path(o) ]
       }
     }
+  }
 
   config("${target_name}_config") {
     visibility = [ ":${invoker.target_name}" ]