Give the Unsafe Browser access to /dev/shm (refs: #15023)

Otherwise it fails to render anything and its window is filled with black.

https://bugzilla.mozilla.org/show_bug.cgi?id=1450169

config/chroot_local-includes/usr/local/lib/tails-shell-library/chroot-browser.sh

@@ -65,7 +65,8 @@ setup_chroot_for_browser () {
     mount -t tmpfs tmpfs "${cow}" && \
     mount -t aufs -o "noatime,noxino,dirs=${aufs_dirs}" aufs "${chroot}" && \
     mount -t proc proc "${chroot}/proc" && \
-    mount --bind "/dev" "${chroot}/dev" || \
+    mount --bind "/dev" "${chroot}/dev" && \
+    mount --bind "/dev/shm" "${chroot}/dev/shm" || \
         return 1
 
     # Workaround for #6110