Add missing dependency of audio_unittests on xdisplaycheck.

audio_unittests.isolate includes xdisplaycheck so audio_unittests should depend
on it.

BUG=416594

Review URL: https://codereview.chromium.org/1137933006

Cr-Commit-Position: refs/heads/master@{#330850}

media/media.gyp

@@ -1964,6 +1964,13 @@
           'sources': [
             'audio_unittests.isolate',
           ],
+          'conditions': [
+            ['use_x11==1', {
+              'dependencies': [
+                '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
+              ],
+            }],
+          ],
         },
       ],
     }],