CHROMIUM: low_mem: fix file_size underflow

Currently we use the following equation to treat some file backed
memory as free meory:

available_file_mem = file_mem - dirty_mem - min_file_mem;

But sometimes file_mem may be less than min_file_mem, which is a
constant.

BUG=chromium:730702
TEST=printed additional debugging info in local testing

Change-Id: Ie5d7015a2609419a1fae4d15647dea229cd4a045
Reviewed-on: https://chromium-review.googlesource.com/527712
Commit-Ready: Ben Cheng <bccheng@chromium.org>
Tested-by: Ben Cheng <bccheng@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/541924
Reviewed-by: Ben Cheng <bccheng@chromium.org>
(cherry picked from commit f255d704d9aa58118c41a8d525d9ef5aaabd3576)
Reviewed-on: https://chromium-review.googlesource.com/557507
Tested-by: Luigi Semenzato <semenzato@chromium.org>
1 file changed