Add support for writing unit tests for Mojo-backed apps/extensions APIs.

extensions/renderer/script_context.cc

Index: extensions/renderer/script_context.cc
diff --git a/extensions/renderer/script_context.cc b/extensions/renderer/script_context.cc
index 0f40f9a4834279424048602849e1f393bf57a5c3..97e6c763be0fd1b3f3ecfea237e437f6a1ae782c 100644
--- a/extensions/renderer/script_context.cc
+++ b/extensions/renderer/script_context.cc
@@ -121,6 +121,8 @@ void ScriptContext::DispatchEvent(const char* event_name,
 }
 
 void ScriptContext::DispatchOnUnloadEvent() {
+  v8::HandleScope handle_scope(isolate());
+  v8::Context::Scope context_scope(v8_context());
   module_system_->CallModuleMethod("unload_event", "dispatch");
 }