Move bookmarks_ui manifest key from chrome_settings_overrides to chrome_ui_overrides

Separates bookmark UI overrides from settings overrides to avoid
future conflict in requirements/goals of the two different pieces of
functionality.

Retains deprecated support under chrome_settings_overrides for
backwards compatibility. This will be removed for the M36 release.

BUG=349049
R=kalman@chromium.org, pkasting@chromium.org, vasilii@chromium.org

Review URL: https://codereview.chromium.org/183883033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255959 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255959