Remove DCHECKs on ack/ack request deserialization.

This allows fuzzing to deserialize arbitrary messages, while still
checking programming errors.

Bug: 1046895
Change-Id: Iafc5418ddbcbf8197ce3991d08c4f8d7bf374234
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031165
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#737091}

mojo/core/ports/event.cc

@@ -405,7 +405,6 @@ UserMessageReadAckRequestEvent::UserMessageReadAckRequestEvent(
     uint64_t sequence_num_to_acknowledge)
     : Event(Type::kUserMessageReadAckRequest, port_name),
       sequence_num_to_acknowledge_(sequence_num_to_acknowledge) {
-  DCHECK(sequence_num_to_acknowledge);
 }
 
 UserMessageReadAckRequestEvent::~UserMessageReadAckRequestEvent() = default;
@@ -438,7 +437,6 @@ UserMessageReadAckEvent::UserMessageReadAckEvent(
     uint64_t sequence_num_acknowledged)
     : Event(Type::kUserMessageReadAck, port_name),
       sequence_num_acknowledged_(sequence_num_acknowledged) {
-  DCHECK(sequence_num_acknowledged);
 }
 
 UserMessageReadAckEvent::~UserMessageReadAckEvent() = default;