Add Observatory to sky dart_controller

mojo/dart/embedder/embedder.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Template to generate a dart embedder package.
 # Required invoker inputs:
 #   String package (relative path to package e.g. mojo/public/interfaces/application)
 template("dart_embedder_package") {
   package = invoker.package
   package_path = "//$package"
@@ skipping 30 matching lines @@
           "$root_gen_dir/dart_embedder_packages/$destination/{{source_file_part}}"),
     ]
   }
 }
 
 # Template to generate a dart embedder resource.cc file.
 # Required invoker inputs:
 #   String output (name of output file)
 #   List inputs (list of input files to be included)
 #   String table_name (name of symbol for resource table)
+#   String root_prefix (base directory of resources)
 # Optional invoker inputs:
 #   String input_directory (directory of resources that are recursively added)
 #   List deps
 #   List datadeps
 template("dart_embedder_resources") {
   action(target_name) {
     script = "//dart/runtime/tools/create_resources.py"
     deps = []
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
     datadeps = []
     if (defined(invoker.datadeps)) {
       datadeps = invoker.datadeps
     }
 
     output = invoker.output
     outputs = [
       output,
     ]
 
     inputs = [ script ] + invoker.inputs
 
+    root_prefix = rebase_path(invoker.root_prefix)
+
     args = [
       "--output",
       rebase_path(output),
       "--outer_namespace",
       "mojo",
       "--inner_namespace",
       "dart",
       "--table_name",
       invoker.table_name,
       "--root_prefix",
-      rebase_path("//mojo/dart/embedder/"),
+      root_prefix,
     ]
     if (defined(invoker.input_directory)) {
       args += [
         "--client_root",
         rebase_path(invoker.input_directory),
       ]
     }
     args += rebase_path(invoker.inputs)
   }
 }