Add meminfo swap metrics.

The current metrics don't show how much swap we're using at a given
time.  We only have indirect measures (page faults rate for instance).

We also need to add one type of histogram scale, since compressed swap
can exceed total RAM, and all current histograms are on a scale from
zero to total RAM.

BUG=chromium-os:38583
TEST=stared at the code really hard and compiled it
BRANCH=none

Change-Id: Icb9dce5efe5dbd78123aad51ffd369cb46721096
Reviewed-on: https://gerrit.chromium.org/gerrit/42598


Reviewed-by: Sonny Rao <sonnyrao@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>