Gerrit plugins built with GWT and gerrit-plugin-gwtui access
non-documented properties and depend on installGwt method's behavior.
This change provides minimal compatible interface to avoid runtime JS
exceptions when loading such plugins into PolyGerrit.
Bug: Issue 5151
Change-Id: Idf3f341a3623f4d3ae8c2f848b0043effa2d2f2d