Update broker socket permssion

This change allows all users in the same group to access external mojo
service.

Bug: 140592503
Test: Build and verified with external client
Change-Id: I1f82378460fd8ae002f02d09103841541b1aa788
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808630Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Jiacheng Guo <gjc@google.com>
Cr-Commit-Position: refs/heads/master@{#697467}

chromecast/external_mojo/public/cpp/external_mojo_broker.cc

@@ -7,6 +7,10 @@
 #include <map>
 #include <utility>
 
+#if OS_LINUX
+#include <sys/stat.h>
+#endif
+
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/location.h"
@@ -393,6 +397,11 @@ ExternalMojoBroker::ExternalMojoBroker(const std::string& broker_path) {
   mojo::PlatformChannelServerEndpoint server_endpoint =
       named_channel.TakeServerEndpoint();
   DCHECK(server_endpoint.is_valid());
+
+#if OS_LINUX
+  chmod(broker_path.c_str(), 0770);
+#endif
+
   read_watcher_ = std::make_unique<ReadWatcher>(
       connector_.get(), server_endpoint.TakePlatformHandle());
 }