Use an even better random number generation scheme in the prune test
Use an even better random number generation scheme in the prune test

base::RandInt(0, max - 1) has a uniform distribution.
base::RandUint64() % max does not.

TEST=crashpad_client_test PruneCrashReports.PruneOrder
R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/1417443002 .

atom/crashpad,atom/crashpad,chromium/crashpad,chromium/crashpad,chromium/crashpad,atom/crashpad