Browser: Turn on multi-process mode by default :^)

Frick it, let's just enable this by default and give ourselves a reason
to improve things! Some things are broken, and there's a bit of flicker
when resizing, but we can do this.

This drastically improves our web browsing security model by isolating
each tab into its own WebContent process that runs as an unprivileged
user with a tight pledge+unveil sandbox.

To get a single-process browser, you can start it with -s.

Userland/Applications/Browser/main.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -51,7 +51,7 @@
 namespace Browser {
 
 String g_home_url;
-bool g_multi_process = false;
+static bool s_single_process = false;
 
 static String bookmarks_file_path()
 {
@@ -78,7 +78,7 @@ int main(int argc, char** argv)
     const char* specified_url = nullptr;
 
     Core::ArgsParser args_parser;
-    args_parser.add_option(Browser::g_multi_process, "Multi-process mode", "multi-process", 'm');
+    args_parser.add_option(Browser::s_single_process, "Single-process mode", "single-process", 's');
     args_parser.add_positional_argument(specified_url, "URL to open", "url", Core::ArgsParser::Required::No);
     args_parser.parse(argc, argv);
 
@@ -180,7 +180,7 @@ int main(int argc, char** argv)
 
     Function<void(URL url, bool activate)> create_new_tab;
     create_new_tab = [&](auto url, auto activate) {
-        auto type = Browser::g_multi_process ? Browser::Tab::Type::OutOfProcessWebView : Browser::Tab::Type::InProcessWebView;
+        auto type = Browser::s_single_process ? Browser::Tab::Type::InProcessWebView : Browser::Tab::Type::OutOfProcessWebView;
         auto& new_tab = tab_widget.add_tab<Browser::Tab>("New tab", type);
 
         tab_widget.set_bar_visible(!window->is_fullscreen() && tab_widget.children().size() > 1);