Enable webkit_unit_tests on TSan built bot

Depending on:
https://chromium-review.googlesource.com/c/518308/

Bug: 727804
Change-Id: I70e48bd5336a2fca3e5661ef1df8e03941003a10
Reviewed-on: https://chromium-review.googlesource.com/517675Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476492}

testing/buildbot/chromium.memory.json

@@ -1499,6 +1499,12 @@
         },
         "test": "views_unittests"
       },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "webkit_unit_tests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true