Ensure ShellExtensionSystem::Init() is called in all extensions_browsertests.

extensions/browser/guest_view/app_view/app_view_apitest.cc

Index: extensions/browser/guest_view/app_view/app_view_apitest.cc
diff --git a/extensions/browser/guest_view/app_view/app_view_apitest.cc b/extensions/browser/guest_view/app_view/app_view_apitest.cc
index 6e7a4df2c9e79f85aaeab029a60a8fc227d61c64..44c96ce550f5d25e898e413010b01cf876b794f5 100644
--- a/extensions/browser/guest_view/app_view/app_view_apitest.cc
+++ b/extensions/browser/guest_view/app_view/app_view_apitest.cc
@@ -120,8 +120,6 @@ class AppViewTest : public AppShellTest,
   void RunTest(const std::string& test_name,
                const std::string& app_location,
                const std::string& app_to_embed) {
-    extension_system_->Init();
-
     const Extension* app_embedder = LoadApp(app_location);
     ASSERT_TRUE(app_embedder);
     const Extension* app_embedded = LoadApp(app_to_embed);