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When settings in a window is enabled, add a back button to the devices
page. This helps because the window has no address bar. See bug for
screenshot.

Longer-term, the devices page should show as an overlay in Settings
rather than load as a new page.

RTL is not enabled for this page; see issue 386020.

BUG=384333
R=noamsml@chromium.org,oshima@chromium.org

+oshima@ for icon. Oshima, is theme_resources the right place for this?
+stevenjb@ for browser shenanigans in local_discovery_ui.cc.

Review URL: https://codereview.chromium.org/338423005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278376 0039d316-1c4b-4281-b951-d872f2087c98