buildbot: Enable webkit_layout_tests on swarming.

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 3472 matching lines @@
         ],
         "isolate_name": "telemetry_unittests",
         "name": "telemetry_unittests",
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true,
           "shards": 4
         }
+      },
+      {
+        "isolate_name": "webkit_layout_tests_exparchive",
+        "merge": {
+          "args": [
+            "--verbose",
+            "--results-json-override-with-build-property",
+            "build_number",
+            "buildnumber",
+            "--results-json-override-with-build-property",
+            "builder_name",
+            "buildername",
+            "--results-json-override-with-build-property",
+            "chromium_revision",
+            "got_revision_cp"
+          ],
+          "script": "//third_party/WebKit/Tools/Scripts/merge-layout-test-results"
+        },
+        "name": "webkit_layout_tests",
+        "results_handler": "layout tests",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Ubuntu-14.04"
+            }
+          ],
+          "shards": 6
+        }
       }
     ],
     "scripts": [
       {
         "name": "checkdeps",
         "script": "checkdeps.py"
       },
       {
         "name": "checkperms",
         "script": "checkperms.py"
@@ skipping 1013 matching lines @@
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true,
           "shards": 4
         }
       }
     ]
   }
 }