Deploy Android Nexus 5X bot on chromium.gpu.

This runs essentially the same tests as the desktop bots.

BUG=786572
NOTRY=true

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;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
Change-Id: I36718069f2a786e3f7e274645d3ff6f58404f492
Reviewed-on: https://chromium-review.googlesource.com/777728
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517650}

content/test/gpu/generate_buildbot_json.py

@@ -227,6 +227,18 @@ WATERFALL = {
       'swarming': True,
       'os_type': 'linux',
     },
+    'Android Release (Nexus 5X)': {
+      'swarming_dimensions': [
+        {
+          'device_type': 'bullhead',
+          'device_os': 'M',
+          'os': 'Android'
+        },
+      ],
+      'build_config': 'android-chromium',
+      'swarming': True,
+      'os_type': 'android',
+    },
   }
 }