Extensions: Clarify ManifestHandler interface expectations.

extensions/common/manifest_handler.cc

Index: extensions/common/manifest_handler.cc
diff --git a/extensions/common/manifest_handler.cc b/extensions/common/manifest_handler.cc
index 24ab931e30da51b0163587a01ab8762849e2d4fa..44b8f64b0319da5e8254acf1cf1f31ee4e497dd1 100644
--- a/extensions/common/manifest_handler.cc
+++ b/extensions/common/manifest_handler.cc
@@ -10,6 +10,7 @@
 
 #include "base/logging.h"
 #include "base/stl_util.h"
+#include "base/threading/thread_restrictions.h"
 #include "extensions/common/extension.h"
 #include "extensions/common/permissions/manifest_permission.h"
 #include "extensions/common/permissions/manifest_permission_set.h"
@@ -90,6 +91,7 @@ bool ManifestHandler::ParseExtension(Extension* extension,
 bool ManifestHandler::ValidateExtension(const Extension* extension,
                                         std::string* error,
                                         std::vector<InstallWarning>* warnings) {
+  base::ThreadRestrictions::AssertIOAllowed();
   return GetRegistry()->ValidateExtension(extension, error, warnings);
 }