Convert gpu_unittests to run exclusively on Swarming

testing/buildbot/chromium.linux.json

 {
   "Android GN": {
     "additional_compile_targets": [
       "chrome_shell_apk",
       "mandoline:all"
     ],
     "gtest_tests": [
       {
         "test": "html_viewer_unittests"
       },
@@ skipping 103 matching lines @@
         },
         "test": "content_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gpu_unittests"
       },
       {
         "test": "ipc_tests"
       },
       {
         "test": "jingle_unittests"
       },
       {
         "test": "media_unittests"
@@ skipping 1041 matching lines @@
       }
     ],
     "scripts": [
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   }
 }