Increase number of interactive_ui_tests shards 1 -> 3 on chromium.memory

This should reduce cycle time difference between linux_chromium_asan_rel
and linux_chromium_asan_rel_ng .

BUG=387743

Review URL: https://codereview.chromium.org/1104983002

Cr-Commit-Position: refs/heads/master@{#327027}

testing/buildbot/chromium.memory.json

@@ -76,7 +76,8 @@
       },
       {
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 3
         },
         "test": "interactive_ui_tests"
       },