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This adds SetDefaultProcessErrorHandler to the public C ABI for Mojo
Core to replace the only remaining non-public, non-Init, non-test-only
API in the mojo::core namespace.

Setting a default process error handler is something the Network Service
uses as part of its own application logic in order to generate useful
crash reports when bad IPCs are received. This suggests that the feature
belongs in the public API, and indeed it must be exposed there in order
to support Chrome loading Mojo Core dynamically in some runtime
environments.

A follow-up CL will transition call sites from the static mojo::core API
to this new public one.

helper in mojo/public/cpp/test_support, which is migrated to the public
API by this CL.

Bug: 1082473
Test: Several content_unittests tests rely on the BadMessageObserver
Change-Id: I77378467afde722b8173df56538221fcaf82a4eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227866
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774869}