[Remoting Linux] Allow native messaging host to specify host config path

This CL allows caller of the me2me NMH to override the path to save the host
config, so that we can have separate host config file when testing NMH against
test gaia environment.

BUG=674685

Review-Url: https://codereview.chromium.org/2581043002
Cr-Commit-Position: refs/heads/master@{#438982}

remoting/host/setup/daemon_controller_delegate_linux.cc

@@ -37,7 +37,15 @@ namespace {
 const char kDaemonScript[] =
     "/opt/google/chrome-remote-desktop/chrome-remote-desktop";
 
+// The name of the command-line switch used to specify the host configuration
+// file to use.
+const char kHostConfigSwitchName[] = "host-config";
+
 base::FilePath GetConfigPath() {
+  base::CommandLine* current_process = base::CommandLine::ForCurrentProcess();
+  if (current_process->HasSwitch(kHostConfigSwitchName)) {
+    return current_process->GetSwitchValuePath(kHostConfigSwitchName);
+  }
   std::string filename =
       "host#" + base::MD5String(net::GetHostName()) + ".json";
   base::FilePath homedir;