chromium.webkit: Fix WebKit Linux Trusty (dbg) builder.

testing/buildbot/chromium.webkit.json

 {
   "WebKit Linux Trusty": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "blink_heap_unittests"
       },
       {
@@ skipping 106 matching lines @@
           "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": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
@@ skipping 534 matching lines @@
         "name": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
         "script": "webkit_python_tests.py"
       }
     ]
   }
 }