[Reland][Chromecast] Add a basic stub for ViewsDelegate

views::Widget no longer checks for a ViewsDelegate, so create a stub so
any widgets created don't cause crashes.

Bug: b/132715995
Test: Run cast_shell_browsertests
Change-Id: Ie9578fbff050da4c5f9f08e717953770578dbdd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615498
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660541}