aura: Allow creating content_shell without views.

This builds content_shell with use_aura=1 and toolkit_views=0.

BUG=none
R=derat@chromium.org, jamescook@chromium.org, jochen@chromium.org

Previously, on this issue:
 * Landed in r229507
 * Speculatively reverted in r229521.
 * Relanded in r229524, since the revert didn't fix anything.
 * Reverted in r229525 because it broken win-blink bots.
 * Committed again in r229671 with an incorrect fix.
 * Reverted in r229683

Review URL: https://codereview.chromium.org/26695007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229704 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229704