linux aura: Use Chrome-default NativeTheme as appropriate.

BUG=320896
R=erg@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/66973007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236066 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236066