Add assertions that the empty Platform::cryptographicallyRandomValues() overrides are not being used.

These implementations are not safe and look scary if not accompanied by an assertion. Also one of the comments was incorrect.

BUG=552749

Review URL: https://codereview.chromium.org/1419293005

Cr-Commit-Position: refs/heads/master@{#359229}
int main(int argc, char** argv) {
  BlinkMediaTestSuite test_suite(argc, argv);

  return base::LaunchUnitTests(
      argc, argv, base::Bind(&BlinkMediaTestSuite::Run,
                             base::Unretained(&test_suite)));
}

int main(int argc, char** argv) {
  BlinkMediaTestSuite test_suite(argc, argv);

  return base::LaunchUnitTests(
      argc, argv, base::Bind(&BlinkMediaTestSuite::Run,
                             base::Unretained(&test_suite)));
}
