Run the webkit layout tests under swarming on linux_chromium_rel_ng.

testing/buildbot/chromium.linux.json

 {
   "Android Arm64 Builder (dbg)": {
     "additional_compile_targets": [
       "all"
     ]
   },
   "Android Builder": {
     "additional_compile_targets": [
       "cronet_test_instrumentation_apk",
       "system_webview_apk"
@@ skipping 3355 matching lines @@
         }
       },
       {
         "isolate_name": "devtools_eslint",
         "name": "devtools_eslint",
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       },
       {
+        "isolate_name": "webkit_layout_tests_exparchive",
+        "name": "webkit_layout_tests_exparchive",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 16
+        }
+      },
+      {
+        "isolate_name": "some_webkit_layout_tests",
+        "name": "some_webkit_layout_tests",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 4
+        }
+      },
+      {
         "isolate_name": "telemetry_gpu_unittests",
         "name": "telemetry_gpu_unittests",
         "override_compile_targets": [
           "telemetry_gpu_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       },
       {
@@ skipping 27 matching lines @@
         "name": "checkdeps",
         "script": "checkdeps.py"
       },
       {
         "name": "checkperms",
         "script": "checkperms.py"
       },
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
+      },
+      {
+        "args": {
+          "shards": 16
+        },
+        "name": "locally_swarmed_webkit_layout_tests",
+        "script": "run_locally_swarmed_webkit_layout_tests.py"
       }
     ]
   },
   "Linux Tests (dbg)(1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
@@ skipping 899 matching lines @@
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       }
     ]
   }
 }