[build] Add missing webkit isolate config

TBR=sergiyb@chromium.org
NOTRY=true
NOPRESUBMIT=true
NOTREECHECKS=true

Bug: chromium:669910
Change-Id: I13ac32f3177677f4cf86602bee4038241933f38e
Reviewed-on: https://chromium-review.googlesource.com/983599Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52271}

infra/mb/gn_isolate_map.pyl

@@ -67,4 +67,8 @@
     "label": "//test/unittests:unittests",
     "type": "script",
   },
+  "webkit": {
+    "label": "//test/webkit:v8_webkit",
+    "type": "script",
+  },
 }
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