Disable all GPU tests on N5X bot temporarily.

This bot config is having stability problems - disable all tests until
the issue is resolved.

BUG=672502
NOTRY=true
NOPRESUBMIT=true
R=kbr@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2562133002
Cr-Commit-Position: refs/heads/master@{#437676}

content/test/gpu/generate_buildbot_json.py

@@ -1432,6 +1432,9 @@ def tester_config_matches_tester(tester_name, tester_config, tc, is_fyi,
   return True
 
 def should_run_on_tester(tester_name, tester_config, test_config, is_fyi):
+  # TODO(jmadill): Re-enable when n5x fixed. See http://crbug.com/672502.
+  if 'Nexus 5X' in tester_name:
+    return False
   # Check if this config is disabled on this tester
   if 'disabled_tester_configs' in test_config:
     for dtc in test_config['disabled_tester_configs']: