Shard browser_tests on Linux ClangToT bots

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/1645753003

Cr-Commit-Position: refs/heads/master@{#372087}

testing/buildbot/chromium.fyi.json

@@ -931,7 +931,8 @@
       },
       {
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 5
         },
         "test": "browser_tests"
       },
@@ -1323,7 +1324,8 @@
       },
       {
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 5
         },
         "test": "content_browsertests"
       },