CHROMIUM: low-mem: improve detection of low-memory conditions.

CHROMIUM: low-mem: improve detection of low-memory conditions.

This patch tries to address issues with the current low-memory detection.
Rather than using the logical conjunction of two separate conditions, we
compute a single number which estimates how much memory is available
for allocation with little or no work.

BUG=chromium-os:20086
TEST=ran kernel_LowMemNotify autotest.  Will expand test (go/crosbug/26853)

Change-Id: Ib47c4ffefac834379003c15b119a38d8ceae94a5
Signed-off-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/16499