Make ColorChooserAura browser modal, not always-on-top.

Parent the chooser widget to the content's browser window.
(ColorChooserView already returns ui::MODAL_TYPE_WINDOW)
Make it no longer always-on-top (has been since r144111).

BUG=295832,295832
TEST=The Color chooser is browser modal, not always on top; see an example at <http://github.hubspot.com/pace/docs/welcome>.
R=mukai@chromium.org,pkasting@chromium.org

Review URL: https://codereview.chromium.org/65733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234615 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234615