Initial size of those tmpfs-mounted directories is calculated as 50% of
RAM at VM startup time. Which happen to be quite small number, like
150M. Having such small /tmp and/or /dev/shm apparently isn't enough for
some applications like Google chrome. So set the size statically at 1GB,
which would be the case for baremetal system with 2GB of RAM.
FixesQubesOS/qubes-issues#1003
