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 79 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wtf_unittests"
       }
     ],
     "isolated_scripts": [
       {
+        "args": [
+          "--debug"
+        ],
         "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",
@@ skipping 557 matching lines @@
         "name": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
         "script": "webkit_python_tests.py"
       }
     ]
   }
 }