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This CL makes a few changes in the direction of removing dependencies
on ErrorCode by pushing them up a layer:

- Trap now simply uses TrapKeys directly to store trap function and
metadata instead of needlessly encoding them as ErrorCode.

- MakeTrap returns a bare trap ID and it's instead the caller's
responsibility to encode in an ErrorCode if needed, which is now
handled by ErrorCode's trap constructor.

- Also, ErrorCodeFromTrapId() is replaced by IsSafeTrapId() to answer
the one question that SandboxBPF was interested in knowing about
existing traps.

- Change a few SandboxBPF trap-constructing functions into static
functions since they don't depend on any SandboxBPF instance member
variables.

BUG=414363

Review URL: https://codereview.chromium.org/572753002

Cr-Commit-Position: refs/heads/master@{#295121}