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

Review URL: https://codereview.chromium.org/988783002

Cr-Commit-Position: refs/heads/master@{#319787}
7 files changed