[Android] Add support for external sharding to the test runner.

testing/buildbot/chromium.linux.json

 {
   "Android Arm64 Builder (dbg)": {
     "additional_compile_targets": [
       "all"
     ]
   },
   "Android Builder": {
     "additional_compile_targets": [
       "cronet_test_instrumentation_apk",
       "system_webview_apk"
@@ skipping 257 matching lines @@
           "hard_timeout": 1800,
           "output_links": [
             {
               "link": [
                 "https://luci-logdog.appspot.com/v/?s",
                 "=android%2Fswarming%2Flogcats%2F",
                 "${TASK_ID}%2F%2B%2Funified_logcats"
               ],
               "name": "shard #${SHARD_INDEX} logcats"
             }
-          ]
+          ],
+          "shards": 2

[jbudorick, 2017/03/11 01:06:37]

This is for demonstration purposes only.

         },
         "test": "chrome_public_test_apk"
       },
       {
         "override_compile_targets": [
           "chrome_sync_shell_test_apk"
         ],
         "override_isolate_target": "chrome_sync_shell_test_apk",
         "swarming": {
           "can_use_on_swarming_builders": true,
@@ skipping 3998 matching lines @@
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true,
           "shards": 4
         }
       }
     ]
   }
 }