[ MimeHandlerView ] Fix the cause for a flakiness

The test PDFExtensionInternalLinkClickTest.ShiftLeft exposed a race
where the embedder RenderFrameHost is nullptr by the time the IPC
ReadyToCreateMimeHandlerView arrives. This CL checks |rfh| to ensure it
is not nullptr.

TBR=wjmaclean@chromium.org

Bug: 967045
Change-Id: Ib80499c8d9e758c8ef07b7d06f69429062d8c6ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1633991Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664098}

extensions/browser/guest_view/extensions_guest_view_message_filter.cc

@@ -140,6 +140,8 @@ void ExtensionsGuestViewMessageFilter::ReadyToCreateMimeHandlerView(
   }
   auto* rfh =
       content::RenderFrameHost::FromID(render_process_id_, render_frame_id);
+  if (!rfh)
+    return;
   if (auto* mhve = MimeHandlerViewEmbedder::Get(rfh->GetFrameTreeNodeId()))
     mhve->ReadyToCreateMimeHandlerView(success);
 }