Increase number of shards for Layout Tests on Dummy WebKit Mac10.13

https://crrev.com/c/996757 left this builder. It is used by the
mac10.13_blink_rel try bot.

Bug: 828543
Change-Id: Ic6017a32994d43419bb16af039454bad1f42e0fc
Reviewed-on: https://chromium-review.googlesource.com/1002919
Reviewed-by: John Budorick <[email protected]>
Commit-Queue: Robert Ma <[email protected]>
Cr-Commit-Position: refs/heads/master@{#549223}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index 9c952f52a..16389fa 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -3537,7 +3537,7 @@
       # chromium.fyi
       'Dummy WebKit Mac10.13': {
         'swarming': {
-          'shards': 6,
+          'shards': 12,
         },
       },
       'linux-blink-heap-incremental-marking': {