[libtxt] Fix the flag used to check validity of the strikeout position in Skia font metrics (#15981)

This reverts commit ff921cd6.

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* changing user limits for macos

* check kernel name instead

MethodChannel and BasicMessageChannel in the C++ wrapper didn't have the
expected semantics that passing a null handler would remove any existing
handler. This was inconsistent with other platforms and with the
lower-level object APIs in this wrapper, and made unregistration in
cases where that's desirable more difficult due to needing to keep other
object references.

Adds tests for this functionality, and some backfill of missing tests
for basic handler behavior.

See https://github.com/flutter/flutter/issues/51207

On Fuchsia, we can now get executable VMOs from trusted backing
filesystems.  This allows us to remove the use of replace_as_executable
in favor of opening files with `fdio_open_fd_at` with the
`OPEN_RIGHT_EXECUTABLE` flag and getting VMOs by calling
`fdio_get_vmo_exec`.

By moving the responsibility for executability into the filesystem, we
should be able to remove deprecated-ambient-replace-as-executable from
component manifests for non-JIT runners (the JIT runners still call
replace_as_executable in Dart's allocator).

Test: verified locally that this works on Astro on a _user build with
the runtime allowlist tightened.

This is a new dependency introduced to the create_sdk build in
https://dart-review.googlesource.com/c/sdk/+/136842

This reverts commit fdabcadd.