testing: increase shard for blink_web_tests on 'Mac10.15 Tests (dbg)'

This is workaround for bot died like
https://chromium-swarm.appspot.com/task?d=true&id=54c5a9a216ec5310

Bug: 1226706
Change-Id: Iad872e44a7d5307c9dfce743604e9975c5dc7e17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3033841
Commit-Queue: Takuto Ikuta <[email protected]>
Auto-Submit: Takuto Ikuta <[email protected]>
Reviewed-by: Ben Pastene <[email protected]>
Cr-Commit-Position: refs/heads/master@{#902892}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index 124e7e1..d929794 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -287,7 +287,7 @@
           '--debug',
         ],
         'swarming': {
-          'shards': 25,
+          'shards': 40,
         },
       },
       'ToTFuchsia x64': {