Make mime handlers inherit the session storage of their embedders.

Mime handlers create a nested WebContents to contain the mime handler
extension. Currently, these run with a separate session storage
namespace, resulting in session storage being cleared if a tab recreates
a mime handler, such as due to a reload. Change mime handler WebContents
creation to inherit the session storage namespaces of their embedders so
the session storage behaves similarly to iframes within a single tab.

Bug: 819761
Change-Id: I3b4f653836190ed86572eb12cdbeef3fbc157683
Reviewed-on: https://chromium-review.googlesource.com/1116633
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570814}

content/browser/web_contents/web_contents_impl.cc

@@ -291,6 +291,12 @@ std::unique_ptr<WebContents> WebContents::CreateWithSessionStorage(
         .SetSessionStorageNamespace(it->first, it->second.get());
   }
 
+  if (params.guest_delegate) {
+    // This makes |new_contents| act as a guest.
+    // For more info, see comment above class BrowserPluginGuest.
+    BrowserPluginGuest::Create(new_contents.get(), params.guest_delegate);
+  }
+
   new_contents->Init(params);
   return new_contents;
 }

extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc

@@ -195,7 +195,11 @@ void MimeHandlerViewGuest::CreateWebContents(
   params.guest_delegate = this;
   // TODO(erikchen): Fix ownership semantics for guest views.
   // https://crbug.com/832879.
-  std::move(callback).Run(WebContents::Create(params).release());
+  std::move(callback).Run(
+      WebContents::CreateWithSessionStorage(
+          params,
+          owner_web_contents()->GetController().GetSessionStorageNamespaceMap())
+          .release());
 
   registry_.AddInterface(
       base::Bind(&MimeHandlerServiceImpl::Create, stream_->GetWeakPtr()));