Allow <webview> to access URLs in the origin of the app embedding it.

extensions/browser/guest_view/web_view/web_view_guest.cc

Index: extensions/browser/guest_view/web_view/web_view_guest.cc
diff --git a/extensions/browser/guest_view/web_view/web_view_guest.cc b/extensions/browser/guest_view/web_view/web_view_guest.cc
index 14917784a30cd27bc5244c1cae4d33572b2642f1..692e490b24e85894f9fd9ce0c30f49204b5521e6 100644
--- a/extensions/browser/guest_view/web_view/web_view_guest.cc
+++ b/extensions/browser/guest_view/web_view/web_view_guest.cc
@@ -364,7 +364,17 @@ void WebViewGuest::CreateWebContents(
       owner_render_process_host->GetBrowserContext(),
       std::move(guest_site_instance));
   params.guest_delegate = this;
-  callback.Run(WebContents::Create(params));
+  WebContents* new_contents = WebContents::Create(params);
+
+  // Grant access to the origin of the embedder to the guest process. This
+  // allows blob:/filesystem: URLs with the embedder origin to be created
+  // inside the guest. It is possible to do this by running embedder code
+  // through webview accessible_resources.
+  content::ChildProcessSecurityPolicy::GetInstance()->GrantOrigin(
+      new_contents->GetMainFrame()->GetProcess()->GetID(),
+      url::Origin(GetOwnerSiteURL()));
+
+  callback.Run(new_contents);
 }
 
 void WebViewGuest::DidAttachToEmbedder() {