crash-reporter: Use C++11 override specifier.

Also remove some unnecessary 'void' arguments.

BUG=None
TEST=`FEATURES=test emerge-$BOARD crash-reporter`

Change-Id: I0039aec93aae14cb238f284fda756b631f9c1587
Reviewed-on: https://chromium-review.googlesource.com/211959


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