mojo/ipc: Drop messages targeting invalid task runner

If the endpoint's task runner does not match the current task runner (or
an equivalent main-thread task runner for the ChannelProxy) when a
executing a message dispatch task for that endpoint, this ignores the
message instead of dispatching it on the wrong sequence.

(cherry picked from commit 0747c9405e1098c0bbbc5593183696eafb45b72c)

Fixed: 1263457
Change-Id: I4f6ba22de3ff8cf3994b748e1ffd0f0aabb17b70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3318601
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#949140}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3328403
Auto-Submit: Ken Rockot <rockot@google.com>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/branch-heads/4664@{#1269}
Cr-Branched-From: 24dc4ee75e01a29d390d43c9c264372a169273a7-refs/heads/main@{#929512}

diff --git a/ipc/ipc_mojo_bootstrap.cc b/ipc/ipc_mojo_bootstrap.cc
index 0a62612..08aabfe 100644
--- a/ipc/ipc_mojo_bootstrap.cc
+++ b/ipc/ipc_mojo_bootstrap.cc

@@ -969,11 +969,14 @@
     if (!client)
       return;
 
+    if (!endpoint->task_runner()->RunsTasksInCurrentSequence() &&
+        !proxy_task_runner_->RunsTasksInCurrentSequence()) {
+      return;
+    }
+
     // Using client->interface_name() is safe here because this is a static
     // string defined for each mojo interface.
     TRACE_EVENT0("mojom", client->interface_name());
-    DCHECK(endpoint->task_runner()->RunsTasksInCurrentSequence() ||
-           proxy_task_runner_->RunsTasksInCurrentSequence());
 
     // Sync messages should never make their way to this method.
     DCHECK(!message.has_flag(mojo::Message::kFlagIsSync));
@@ -1003,11 +1006,14 @@
     if (!client)
       return;
 
+    if (!endpoint->task_runner()->RunsTasksInCurrentSequence() &&
+        !proxy_task_runner_->RunsTasksInCurrentSequence()) {
+      return;
+    }
+
     // Using client->interface_name() is safe here because this is a static
     // string defined for each mojo interface.
     TRACE_EVENT0("mojom", client->interface_name());
-    DCHECK(endpoint->task_runner()->RunsTasksInCurrentSequence() ||
-           proxy_task_runner_->RunsTasksInCurrentSequence());
     MessageWrapper message_wrapper = endpoint->PopSyncMessage(message_id);
 
     // The message must have already been dequeued by the endpoint waking up