depthcharge: Fix 64-bit portability issues

BUG=None
BRANCH=None
TEST=Compiles successfully

Change-Id: Ib3a6b7d56ed8ad8ef98b8ba61f4cf06ec2578dfa
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://chromium-review.googlesource.com/204453
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Tested-by: Furquan Shaikh <furquan@chromium.org>