[ChromeDriver] Give warning if exclude user-data-dir

ChromeDriver relies on the user-data-dir switch for its operations,
and ignores attempts by the client to exclude this switch. This CL
adds a warning message if the client attempts to exclude user-data-dir.

Bug: chromedriver:1981
Change-Id: I733f16fbdb6bea5dcf130ce7ad97664036f689be
Reviewed-on: https://chromium-review.googlesource.com/662598
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501303}

chrome/test/chromedriver/chrome_launcher.cc

@@ -144,6 +144,8 @@ Status PrepareCommandLine(uint16_t port,
   }
   switches.SetFromSwitches(capabilities.switches);
 
+  if (capabilities.exclude_switches.count("user-data-dir") > 0)
+    LOG(WARNING) << "excluding user-data-dir switch is not supported";
   base::FilePath user_data_dir_path;
   if (switches.HasSwitch("user-data-dir")) {
     user_data_dir_path = base::FilePath(