service_manager: Only start the service manager thread in the browser process.

The browser process is the only embedder process that needs a
service manager thread.

With this change we avoid spinning up an unnecessary I/O thread (and
presumably, once issue 729596 is fixed, a service manager) in
processes that are neither subprocesses nor the browser process,
such as the cloud print service process.

Bug: 855193
Change-Id: Iffae3405f6dd541ce898ac2033c9c10daefd0031
Reviewed-on: https://chromium-review.googlesource.com/1117473
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571640}

services/service_manager/embedder/main.cc

@@ -449,7 +449,7 @@ int Main(const MainParams& params) {
     case ProcessType::kEmbedder:
       if (delegate->IsEmbedderSubprocess())
         CommonSubprocessInit();
-      else {
+      if (command_line.GetSwitchValueASCII(switches::kProcessType).empty()) {
         // TODO(https://crbug.com/729596): Use this task runner to start
         // ServiceManager.
         scoped_refptr<base::SingleThreadTaskRunner> task_runner =