Add webapk_runtime_library_junit_tests to bots.

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 636 matching lines @@
         "test": "junit_unit_tests"
       },
       {
         "test": "net_junit_tests"
       },
       {
         "test": "ui_junit_tests"
       },
       {
         "test": "webapk_client_junit_tests"
+      },
+      {
+        "test": "webapk_runtime_library_junit_tests"
       }
     ],
     "scripts": [
       {
         "name": "webview_licenses",
         "script": "webview_licenses.py"
       }
     ]
   },
   "Android Tests (dbg)": {
@@ skipping 1597 matching lines @@
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       }
     ]
   }
 }