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BPF sandbox policies normal use this code sequence if they are using a
broker:
  auto* broker_process = SandboxLinux::GetInstance()->broker_process();
  if (broker_process->IsSyscallAllowed(sysno)) {
    return Trap(BrokerProcess::SIGSYS_Handler, broker_process);
  }

Switch this to:
  auto* sandbox_linux = SandboxLinux::GetInstance();
  if (sandbox_linux->ShouldBrokerHandleSyscall(sysno))
    return sandbox_linux->HandleViaBroker();

...which has the advantage of being clearer.

This also makes it easier to land SECCOMP_RET_USER_NOTIF support, as
if USER_NOTIF is supported we will use bpf_dsl::UserNotif instead of
bpf_dsl::Trap, and users of the sandbox shouldn't have to care which
one is used.

Bug: 1117351
Change-Id: I809fdb4118fef39d8b142fdd571743c49e0812a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377422
Commit-Queue: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808573}