[wrapper-tracing] Simplify tracing by forbidding tracing from within mixins

We don't have a single use case where we would require wrapper tracing from
within mixins. Remove the infra that would be needed, including the catch-all
handler.

Note that we can now statically check not tracing from within a mixin using a
static_assert.

BUG=chromium:468240

Review-Url: https://codereview.chromium.org/2595053002
Cr-Commit-Position: refs/heads/master@{#440389}
3 files changed