Fix None target type with Ninja build.

GYP doesn't produce a valid Ninja rule for 'none' target type.
These targets are used to make fake dependencies.

Here is a minimal GYP file to show the issue:

{
  'targets': [
    {
      'target_name': 'dummy',
      'type': 'none',
      'sources': [
        'test.h',
      ],
    },
  ],
}

There is a dummy.ninja file, but there is no rules named 'dummy'.
When using the 'msvs-ninja' build, MSVS complains about a missing target
and fails to compile the project.

Patch from etienneb@chromium.org.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/203303011

git-svn-id: http://gyp.googlecode.com/svn/trunk@1883 78cadc50-ecff-11dd-a971-7dbc132099af