Emit error if unknown switch is passed to mojo_shell

mojo/shell/switches.h

Index: mojo/shell/switches.h
diff --git a/mojo/shell/switches.h b/mojo/shell/switches.h
index 3381eeb1ee70b1b1cdb2d8833d21410a0b195af4..e5049262bb1fae845a959e743c6287a120da8b5d 100644
--- a/mojo/shell/switches.h
+++ b/mojo/shell/switches.h
@@ -5,6 +5,9 @@
 #ifndef MOJO_SHELL_SWITCHES_H_
 #define MOJO_SHELL_SWITCHES_H_
 
+#include <set>
+#include <string>
+
 namespace switches {
 
 // All switches in alphabetical order. The switches should be documented
@@ -21,6 +24,8 @@ extern const char kOrigin[];
 extern const char kSpy[];
 extern const char kURLMappings[];
 
+extern const std::set<std::string> GetAllSwitches();
+
 }  // namespace switches
 
 #endif  // MOJO_SHELL_SWITCHES_H_