The diff view file select used its own implementation of a dropdown,
which also had two versions, a native select for mobile and a custom
design for desktop.
gr-autocomplete-dropdown was designed so that it could be used in this
scenario as well, as it also has a native select built in.
This will also allow us to add comments in the dropdown (follow-up
change) and use Gerrit.Nav instead of generated URLs.
Change-Id: I5613bb8e327a3f6ae227bf69e64bfafbbfe09b6c
fde2da6dc2