In attempting to run the media_unittests as isolated tests (for  issue 51880 ), it reports "xdisplaycheck" is missing.

Steps:
1) gn clean out/GN
2) echo gn > out/GN/mb_type (required by [1])
3) tools/mb/mb.py run out/GN media_unittests

Results:
ninja: Entering directory `out/GN'
[1/1] Regenerating ninja files
[4995/4995] LINK ./media_unittests
python tools/swarming_client/isolate.py check -i out/GN/media_unittests.isolate -s out/GN/media_unittests.isolated
Failed to find an input file: Input file /usr/local/google/home/jrummell/src/out/GN/xdisplaycheck doesn't exist
  -> returned 1

Expected Output:
media_unittests run.

Workaround:
Building "xdisplaycheck" before step 3 works.

Investigation:
- BUILD.gn in the root directory contains "if (use_x11) { deps += [ "//tools/xdisplaycheck" ] }" [2].
- media/media_unittests.isolate only includes xdisplaycheck if "use_x11==1" [3].
- "use_x11" is not specified in the args for gn. My settings are "proprietary_codecs = true is_chrome_branded = true ffmpeg_branding = "Chrome" is_debug = true use_goma = true enable_mojo_media = true enable_test_mojo_media_client = true"

[1] "How to run a test isolated locally?" on http://www.chromium.org/developers/testing/isolated-testing/for-swes
[2] https://code.google.com/p/chromium/codesearch#chromium/src/BUILD.gn&q=xdisplaycheck&sq=package:chromium&type=cs&l=254
[3] https://code.google.com/p/chromium/codesearch#chromium/src/media/media_unittests.isolate&q=xdisplaycheck&sq=package:chromium&type=cs&l=36