Get mojo_shell building inside chromium checkout.

mojo/public/mojo_sdk.gni

 # Copyright 2014 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.
 
 # The absolute path to the directory containing the mojo public SDK (i.e., the
 # directory containing mojo/public). The build files within the Mojo public
 # SDK use this variable to allow themselves to be parameterized by the location
 # of the public SDK within a client repo.
 mojo_root = get_path_info("../..", "abspath")
 
@@ skipping 119 matching lines @@
     }
     if (defined(invoker.mojo_sdk_deps)) {
       foreach(sdk_dep, invoker.mojo_sdk_deps) {
         # Check that the SDK dep was not mistakenly given as an absolute path.
         assert(get_path_info(sdk_dep, "abspath") != sdk_dep)
         deps += [ rebase_path(sdk_dep, ".", mojo_root) ]
       }
     }
   }
 }