linux_aura: Only call MenuConfig::InitAura() when using NativeThemeAura.

This makes menu_config_views.cc match menu_config_win.cc; we should only
be loading the aura desktop theme if we're using NativeThemeAura.

BUG=318399
R=sky@chromium.org

Review URL: https://codereview.chromium.org/70533003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234906 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234906