Bump @polymer/polymer from 3.5.1 to 3.5.2 in /webapp

Bumps [@polymer/polymer](https://github.com/Polymer/polymer) from 3.5.1 to 3.5.2.
- [Release notes](https://github.com/Polymer/polymer/releases)
- [Changelog](https://github.com/Polymer/polymer/blob/master/CHANGELOG.md)
- [Commits](Polymer/polymer@v3.5.1...v3.5.2)

---
updated-dependencies:
- dependency-name: "@polymer/polymer"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

webapp/package.json

@@ -86,7 +86,7 @@
     "@polymer/paper-toast": "3.0.1",
     "@polymer/paper-toggle-button": "3.0.1",
     "@polymer/paper-tooltip": "3.0.1",
-    "@polymer/polymer": "3.5.1",
+    "@polymer/polymer": "3.5.2",
     "@vaadin/vaadin-context-menu": "4.5.0",
     "@vaadin/vaadin-date-picker": "4.4.1",
     "@vaadin/vaadin-grid": "5.7.13",