Add an initial implementation of the downloads MD settings page.
Note that the "Change" button for selecting the downloads path doesn't do
anything meaningful yet. We still need to figure out how to call into the
C++ layer to display the appropriate picker.
BUG=
464785
Committed:
https://crrev.com/61f613741603561b14ddf497f110848d592c626a
Cr-Commit-Position: refs/heads/master@{#319787}