metrics: use abs() for integer only.

This cast the type of the paramenter of abs() to integer before
calling the abs() function.

BUG=chromium:352331
TEST=FEATURES="test" emerge-amd64-generic platform2 passes.

Change-Id: Id8b994b743345456f14194b45f3a288b7e3b74f3
Reviewed-on: https://chromium-review.googlesource.com/189849


Reviewed-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>