Added deprecation warning for --single_process

Added warning and exit when attempting to use the single_process
flag to run the tests and launcher in a single process.


Bug: 460513
Change-Id: I344f4477580ed81490002cd13ef5f5e4ed7a021d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894757Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711862}

content/public/test/test_launcher.cc

@@ -265,6 +265,13 @@ int LaunchTests(TestLauncherDelegate* launcher_delegate,
   const base::CommandLine* command_line =
       base::CommandLine::ForCurrentProcess();
 
+  // TODO(tluk) Remove deprecation warning after a few releases. Deprecation
+  // warning issued version 79.
+  if (command_line->HasSwitch("single_process")) {
+    fprintf(stderr, "use --single-process-tests instead of --single_process");
+    exit(1);
+  }
+
   if (command_line->HasSwitch(switches::kHelpFlag)) {
     PrintUsage();
     return 0;