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}

mojo/core/channel.h

@@ -369,6 +369,9 @@ class MOJO_SYSTEM_IMPL_EXPORT Channel
   // Allows the caller to change the Channel's HandlePolicy after construction.
   void set_handle_policy(HandlePolicy policy) { handle_policy_ = policy; }
 
+  // Allows the caller to determine the current HandlePolicy.
+  HandlePolicy handle_policy() const { return handle_policy_; }
+
   // Request that the channel be shut down. This should always be called before
   // releasing the last reference to a Channel to ensure that it's cleaned up
   // on its I/O task runner's thread.
@@ -411,9 +414,6 @@ class MOJO_SYSTEM_IMPL_EXPORT Channel
 
   Delegate* delegate() const { return delegate_; }
 
-  // Allows the caller to determine the current HandlePolicy.
-  HandlePolicy handle_policy() const { return handle_policy_; }
-
   // Called by the implementation when it wants somewhere to stick data.
   // |*buffer_capacity| may be set by the caller to indicate the desired buffer
   // size. If 0, a sane default size will be used instead.

mojo/core/node_channel.cc

@@ -759,11 +759,17 @@ void NodeChannel::OnChannelMessage(
         if (payload_size <= sizeof(Header) + sizeof(data))
           break;
 
+        Channel::HandlePolicy handle_policy;
+        {
+          base::AutoLock lock(channel_lock_);
+          handle_policy = channel_->handle_policy();
+        }
+
         const void* message_start = reinterpret_cast<const uint8_t*>(payload) +
                                     sizeof(Header) + sizeof(data);
         Channel::MessagePtr message = Channel::Message::Deserialize(
             message_start, payload_size - sizeof(Header) - sizeof(data),
-            Channel::HandlePolicy::kAcceptHandles, from_process);
+            handle_policy, from_process);
         if (!message) {
           DLOG(ERROR) << "Dropping invalid relay message.";
           break;