metrics: Use integer types from stdint.h

This CL replaces the deprecated int* and uint* types from
'base/basictypes.h' with the int*_t and uint*_t types from 'stdint.h'.

BUG=chromium:401356
TEST=`FEATURES=test emerge-$BOARD metrics`

Change-Id: Ie5a69edba2c8a9d5185bbc548ed70a5b121c3e3b
Reviewed-on: https://chromium-review.googlesource.com/211381


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>