Create --disable-extensions-except switch

chrome/browser/extensions/extension_system_impl.h

Index: chrome/browser/extensions/extension_system_impl.h
diff --git a/chrome/browser/extensions/extension_system_impl.h b/chrome/browser/extensions/extension_system_impl.h
index 5cd96a310ff1c93fcefe1abbea79f166005cdf8e..16258ae5f3d04cdc297a390bff509fcddeeed79c 100644
--- a/chrome/browser/extensions/extension_system_impl.h
+++ b/chrome/browser/extensions/extension_system_impl.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/command_line.h"
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "extensions/browser/extension_system.h"
@@ -138,6 +139,9 @@ class ExtensionSystemImpl : public ExtensionSystem {
 #endif
 
     OneShotEvent ready_;
+    void LoadExtensionsFromCommandLineFlag(

[Devlin, 2016/07/27 17:01:00]

Two notes here:
If we were to add this method, we should follow the declaration order style:
https://google.github.io/styleguide/cppguide.html#Declaration_Order (at least
for the most part).  Specifically, methods before members.
That said, I think we can probably do away with this method.  See .cc file.

[catmullings, 2016/08/04 22:59:31]

Moved this definition to extension_service.h. 

[catmullings, 2016/08/04 22:59:31]

Done.

+        const base::CommandLine* command_line,
+        const char* switch_name);
   };
 
   Profile* profile_;