CHROMIUM: mm: fix divide error in smaps_account

For swapped out page, its mapcount is zero. Don't divide pss by it
in this case.

BUG=b:135658425
TEST=Built and booted

Change-Id: I41839b22126d5f4ae9ddb3341ae315f73c3e7b1e
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1676697


Tested-by: Yu Zhao <yuzhao@chromium.org>
Commit-Ready: Yu Zhao <yuzhao@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Yu Zhao <yuzhao@chromium.org>