[EoS] Hook up touchless context menus.

This adds a TouchlessExploreSitesPage that knows what to do in case
a touchless modal dialog needs to be shown.

Bug: 948226
Change-Id: I1d23812d83ca15f15b620eb25d3726da7f7c19da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1579914
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653698}
8 files changed