Revert of Simplify mojo_shell since it's now only used for Mandoline.

mojo/shell/switches.h

Index: mojo/shell/switches.h
diff --git a/mojo/shell/switches.h b/mojo/shell/switches.h
index ff2eb6c95dc8f4dff0d3657afd20715f5b13fc3c..52267fec80aa35f39bac2611d1c536e0a086d13b 100644
--- a/mojo/shell/switches.h
+++ b/mojo/shell/switches.h
@@ -11,7 +11,9 @@
 namespace switches {
 
 // All switches in alphabetical order. The switches should be documented
-// alongside the definition of their values in the .cc file.
+// alongside the definition of their values in the .cc file and, as needed,
+// in mojo_main's Usage() function.
+extern const char kArgsFor[];
 extern const char kChildProcess[];
 extern const char kContentHandlers[];
 extern const char kDisableCache[];
@@ -23,6 +25,9 @@ extern const char kMapOrigin[];
 extern const char kOrigin[];
 extern const char kPredictableAppFilenames[];
 extern const char kTraceStartup[];
+extern const char kURLMappings[];
+
+extern const std::set<std::string> GetAllSwitches();
 
 }  // namespace switches