Welcome WebUI: Dependency cleanups and moving nux.AppProxy interface.

 - Add missing import in welcome_app.html.
 - Remove unnecessary i18n_setup.html import in google_app_proxy.html
 - Move nux.AppProxy interface to the only placed it is actually used
   and rename to nux.GoogleAppProxy. This interface was previously used
   by more places, but not the case anymore.

These were discovered while attempting to convert chrome://welcome
to JS modules and Polymer3.

Bug: 965770
Change-Id: Id1c0be2a8174e288f329276e2ed2f6bebc447122
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1690745
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675411}
10 files changed