ash: Add missing xdisplaycheck dependency for tests

BUG=465344
TEST=linux_chromium_clobber_rel_ng does not block new ash_unittests from landing

Review URL: https://codereview.chromium.org/985853004

Cr-Commit-Position: refs/heads/master@{#319786}

ash/BUILD.gn

@@ -395,3 +395,5 @@ test("ash_shell_unittests") {
     deps += [ "//ui/display" ]
   }
 }
+# When adding support for isolates, please have a look at run-time dependencies
+# in the ash_unittests_run target in ash.gyp.

ash/ash.gyp

@@ -1291,6 +1291,15 @@
           'sources': [
             'ash_unittests.isolate',
           ],
+          'conditions': [
+            ['use_x11==1',
+              {
+                'dependencies': [
+                  '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
+                ],
+              }
+            ],
+          ],
         },
       ],
     }],