[Files.app] when a modal dialog is shown, focus to a button on the dialog

BUG=290089
TEST=manually tested
R=hirono@chromium.org

Review URL: https://codereview.chromium.org/26377003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227494 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227494
Moved onc_constants.* from chromeos/network/onc to components/onc.

BUG=267667

Review URL: https://codereview.chromium.org/24600003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227495