Bump browser_tests on linux msan bot to 15 shards

It was bumped from 10 to 12 shards recently in crrev.com/c/3893236.
But the bot is back to regularly timing out some of its shards, eg:
https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20MSan%20Tests/35638/overview
https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20MSan%20Tests/35639/overview

So this bumps it _again_. Theoretically the shards have minimal
overhead. And there's no matching CQ bot for this CI bot. So capacity
hit should be minimal.

Bug: 1376264
Change-Id: Idecb9d003d03609af5c1e2b8d3e633766372f7a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3972238
Reviewed-by: Struan Shrimpton <[email protected]>
Commit-Queue: Ben Pastene <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1062276}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index d612a66..c51c170 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -908,7 +908,7 @@
       },
       'Linux MSan Tests': {
         'swarming': {
-          'shards': 12,
+          'shards': 15,
         },
       },
       'Linux Tests': {