gpu: Add missing dependency on xdisplaycheck.

gpu/gpu.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'nacl_win64_target': 0,
   },
   'includes': [
     'gpu_common.gypi',
@@ skipping 732 matching lines @@
           'type': 'none',
           'dependencies': [
             'gpu_unittests',
           ],
           'includes': [
             '../build/isolate.gypi',
           ],
           'sources': [
             'gpu_unittests.isolate',
           ],
+          'conditions': [
+            ['use_x11==1',
+              {
+                'dependencies': [
+                  '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
+                ],
+              }
+            ],
+          ],
         },
       ],
     }],
   ],
 }