Increase swarming shards to have 15 mins average shard duration in CQ

Recently, I saw some tests has large shard duration.
http://shortn/_XYVwYNWZJb

This CL increases the number of shard so that tests in CQ has 15 minutes average shard duration.

Change-Id: If4011de7a6c3c6f98e98bc70ba2fa9a87aca8f90
Reviewed-on: https://chromium-review.googlesource.com/1104277
Reviewed-by: John Budorick <[email protected]>
Commit-Queue: Takuto Ikuta <[email protected]>
Cr-Commit-Position: refs/heads/master@{#567993}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index 5432c40..95f7a15 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -2413,7 +2413,7 @@
       },
       'Linux TSan Tests': {
         'swarming': {
-          'shards': 5,
+          'shards': 7,
         },
       },
     },