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.

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

git-svn-id: http://src.chromium.org/svn/trunk/src/build@229671 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

build_config.h

@@ -31,7 +31,7 @@
 #elif defined(__linux__)
 #define OS_LINUX 1
 // Use TOOLKIT_GTK on linux if TOOLKIT_VIEWS isn't defined.
-#if !defined(TOOLKIT_VIEWS) && defined(USE_X11)
+#if !defined(TOOLKIT_VIEWS) && defined(USE_X11) && !defined(USE_AURA)
 #define TOOLKIT_GTK
 #endif
 #if defined(__GLIBC__) && !defined(__UCLIBC__)