Allow disabling upload rate-limiting in crashpad_handler

This is a temporary measure to try to account for lower than expected
upload volume from Chrome in the wild. So this doesn't fix bug 23, but
is related. The ability to delimit the upload rate is useful when
testing locally too.

R=mark@chromium.org
BUG=crashpad:23

Review URL: https://codereview.chromium.org/1563683002 .
4 files changed