Fix the mojo_shell window not coming up sometimes on Windows when using --enable-multiprocess.

The problem was that ShowWindow that was eventually called through PlatformViewportWin::Show wasn't working because it was using the default from STARTUPINFO which was false.

Review URL: https://codereview.chromium.org/1106133002

Cr-Commit-Position: refs/heads/master@{#327048}

mojo/shell/child_process_host.cc

@@ -122,7 +122,6 @@ bool ChildProcessHost::DoLaunch() {
 
   base::LaunchOptions options;
 #if defined(OS_WIN)
-  options.start_hidden = true;
   options.handles_to_inherit = &handle_passing_info;
 #elif defined(OS_POSIX)
   options.fds_to_remap = &handle_passing_info;