Update isolate files for swarming tests

Xvfb is needed for the screen capture tests in modules_unittests,
which also brings in xdisplaycheck used by testing/xvfb.py.

libjingle_media_unittest was missing a resource video in the .isolate
file.

BUG=chromium:497757
R=stip@chromium.org

Review URL: https://codereview.webrtc.org/1415603005 .

Cr-Commit-Position: refs/heads/master@{#10365}

setup_links.py

@@ -75,6 +75,7 @@ DIRECTORIES = [
   'tools/valgrind',
   'tools/vim',
   'tools/win',
+  'tools/xdisplaycheck',
 ]
 
 from sync_chromium import get_target_os_list