Add delegate to install web_contents helpers for MimeHandlerView.

extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc

Index: extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
diff --git a/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc b/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
index c65d61459f7e12daac1b1b3c449c494793929941..bba4c871701e352a72c36952c719ecd87fde516c 100644
--- a/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
+++ b/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
@@ -8,9 +8,11 @@
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/common/url_constants.h"
+#include "extensions/browser/api/extensions_api_client.h"
 #include "extensions/browser/guest_view/guest_view_constants.h"
 #include "extensions/browser/guest_view/guest_view_manager.h"
 #include "extensions/browser/guest_view/mime_handler_view/mime_handler_view_constants.h"
+#include "extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest_delegate.h"
 #include "extensions/common/feature_switch.h"
 #include "extensions/strings/grit/extensions_strings.h"
 #include "net/base/url_util.h"
@@ -35,7 +37,9 @@ GuestViewBase* MimeHandlerViewGuest::Create(
 MimeHandlerViewGuest::MimeHandlerViewGuest(
     content::BrowserContext* browser_context,
     int guest_instance_id)
-    : GuestView<MimeHandlerViewGuest>(browser_context, guest_instance_id) {
+    : GuestView<MimeHandlerViewGuest>(browser_context, guest_instance_id),
+      delegate_(ExtensionsAPIClient::Get()->CreateMimeHandlerViewGuestDelegate(
+          this)) {
 }
 
 MimeHandlerViewGuest::~MimeHandlerViewGuest() {
@@ -94,6 +98,11 @@ void MimeHandlerViewGuest::DidAttachToEmbedder() {
       std::string());
 }
 
+void MimeHandlerViewGuest::DidInitialize() {
+  if (delegate_)
+    delegate_->AttachHelpers();
+}
+
 void MimeHandlerViewGuest::HandleKeyboardEvent(
     WebContents* source,
     const content::NativeWebKeyboardEvent& event) {