Prepend "user:" to chmod command

macOS's extended chmod syntax can be used for either users or groups,
and supports "user:<user>" and "group:<group>" to resolve ambiguities.
This causes problems if the username is "user", as chmod can't parse
the ACL in this case. This CL prepends "user:" to all ACLs to avoid
this problem.

Change-Id: I15c143b94a2c3113f518ed2b558f007ad769d0c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088397
Auto-Submit: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748771}

remoting/host/mac/host_service_main.cc

@@ -280,7 +280,7 @@ bool HostService::Enable() {
   // uses the chmod binary to do so.
   base::CommandLine chmod_cmd(base::FilePath("/bin/chmod"));
   chmod_cmd.AppendArg("+a");
-  chmod_cmd.AppendArg(GetUsername() + ":allow:read");
+  chmod_cmd.AppendArg("user:" + GetUsername() + ":allow:read");
   chmod_cmd.AppendArgPath(config_file_);
   std::string output;
   if (!base::GetAppOutputAndError(chmod_cmd, &output)) {