Mark histogram arguments as unused to avoid warnings

When embedding histogram macros, (void) the arguments to avoid "variable
initialized but unused" warnings.

R=mark@chromium.org
BUG=crashpad:100

Change-Id: I97804a01a8b3ca7e786a970e235c09d554e3c3c6
Reviewed-on: https://chromium-review.googlesource.com/380481
Reviewed-by: Mark Mentovai <mark@chromium.org>