Move cygwin_dir to common.gypi.

dart/utils/apidoc/apidoc.gyp

Index: dart/utils/apidoc/apidoc.gyp
diff --git a/dart/utils/apidoc/apidoc.gyp b/dart/utils/apidoc/apidoc.gyp
index aafad1e1c8ddcebdd46d06cc69a26241a5651521..5ce55f83ab543ca565bd7755f5adf4a5cefb89c9 100644
--- a/dart/utils/apidoc/apidoc.gyp
+++ b/dart/utils/apidoc/apidoc.gyp
@@ -3,11 +3,6 @@
 # BSD-style license that can be found in the LICENSE file.
 
 {
-  'variables': {
-    # These variables are used in the creation of the .vcproj file on
-    # Windows.
-    'cygwin_dir': '../../third_party/cygwin',
-  },
   'targets': [
     {
       'target_name': 'api_docs',
@@ -16,11 +11,6 @@
         '../../frog/dart-frog.gyp:frog',
         '../../runtime/dart-runtime.gyp:dart',
       ],
-      'conditions': [
-        ['OS=="win"', {
-          'msvs_cygwin_dirs': ['<(cygwin_dir)'],
-        }],
-      ],
       'actions': [
         {
           'action_name': 'run_apidoc',