Fix dangling pointer in media_router_mojo_impl.cc

This patch fixes an untraiged dangling pointer in
media_router_mojo_impl.cc by switching to base::Unretained and
adding a comment explaining why it is safe: we check that the
pointer is still in the routes_query_ list before using in the
callback method.

Bug: 1381416
Change-Id: I539b0f656fef4255d39de13693529c25eab3c481
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4021642
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Muyao Xu <muyaoxu@google.com>
Cr-Commit-Position: refs/heads/main@{#1071400}