Justify RAW_PTR_EXCLUSION for NativeMessageBuiltInHost

The pointer member of NativeMessageBuiltInHost only ever points at
statically-allocated arrays of strings, and thus isn't at risk of
becoming a dangling pointer. Thus we don't need to incur the overhead
of replacing it with a raw_ptr.

Bug: 346693597
Change-Id: I1793d3e3a12e7031f3f22c43b477d00fb854caab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6089013
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>
Cr-Commit-Position: refs/heads/main@{#1395415}