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

testing/buildbot/chromium.memory.json

 {
   "Linux ASan LSan Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 58 matching lines @@
         "test": "gfx_unittests"
       },
       {
         "test": "google_apis_unittests"
       },
       {
         "test": "gpu_unittests"
       },
       {
         "swarming": {
-          "can_use_on_swarming_builders": true
+          "can_use_on_swarming_builders": true,
+          "shards": 3
         },
         "test": "interactive_ui_tests"
       },
       {
         "test": "ipc_tests"
       },
       {
         "test": "jingle_unittests"
       },
       {
@@ skipping 427 matching lines @@
       },
       {
         "test": "ui_base_unittests"
       },
       {
         "test": "url_unittests"
       }
     ]
   }
 }