[fuchsia] Route diagnostics dictionary to Chromium

Update the CML to route from parent/diagnostics instead of from parent.
LogSink and InspectSink will be routed through diagnostics dictionaries
in the future, and this change is needed in order to allow for a
soft-transition to dictionary capabilities rather than directly from
parent.

It's copied from https://crrev.com/c/6354008 to Crashpad. This change
needs to go to crashpad first before merging to Chromium.

Bug: 376922343
Change-Id: I76d388827c3a560b1f9dd88168e60458fedda7b8
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/6388762
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Zijie He <zijiehe@google.com>