Fix libfuzzer RELAY_EVENT_MESSAGE failures

This change passes in the handle policy to
Channel::Message::Deserialize() to facilitate testing mojo with
libfuzzer. To facilitate this, handle_policy() was moved from being
a protected method to being public in channel so it can be called
and passed into Deserialize().

Validated the fuzzer no longer crashes, instead we get:
[1015/100038.256:ERROR:channel.cc(391)] Rejecting message with
unexpected handle attachments.
[1015/100038.260:ERROR:node_channel.cc(789)] Dropping invalid relay
message.
[1015/100038.261:ERROR:node_channel.cc(863)] Received invalid
message type: 9 closing channel.

bug: 40241071
Change-Id: Ie7c6e0f11f5e30654a8a9baeb061c6318a583b77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5932529
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Emily Andrews <emiled@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1372495}