linux-blink-animation-use-time-delta bot: second attempt to fix webkit_layout_tests

The bot currently times out on webkit_layout_tests. I noticed that most
other bots use 'chromium_webkit_isolated_scripts' rather than
'webkit_layout_tests_isolated_scripts'. The former has a swarming size
argument of 12, whilst the latter has none (and thus seems to use a
swarm of size 1).

Hopefully switching to 'chromium_webkit_isolated_scripts' will enable
the bot to finish the tests.

Bug: 876029
Change-Id: I35c77992b28e5a108bee65c1ea91bb39d66c9bd2
Reviewed-on: https://chromium-review.googlesource.com/1197267Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587864}

testing/buildbot/chromium.fyi.json

@@ -3697,7 +3697,8 @@
         "name": "webkit_layout_tests",
         "results_handler": "layout tests",
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 12
         }
       }
     ]

testing/buildbot/waterfalls.pyl

@@ -2481,7 +2481,7 @@
         ],
         'test_suites': {
           'gtest_tests': 'chromium_gtests',
-          'isolated_scripts': 'webkit_layout_tests_isolated_scripts',
+          'isolated_scripts': 'chromium_webkit_isolated_scripts',
         },
       },
       'linux-annotator-rel': {