Don't abort the MimeHandlerStreamManager stream if we receive a RenderProcessHostChanged notification for a null old host.

This indicates that a subframe is being created. If we ignore  the old_host here and set
the new host pointer in the MimeHandlerStreamManager::EmbedderObserver class, this causes
the stream to be aborted prematurely leading to embedded PDFs not loading.

This should fix bug 650985

BUG=650985

Review-Url: https://codereview.chromium.org/2418963002
Cr-Commit-Position: refs/heads/master@{#425498}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 2d93f2d..964b243 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -309,6 +309,8 @@
 WebContents* WebContents::FromFrameTreeNodeId(int frame_tree_node_id) {
   FrameTreeNode* frame_tree_node =
       FrameTreeNode::GloballyFindByID(frame_tree_node_id);
+  if (!frame_tree_node)
+    return nullptr;
   return FromRenderFrameHost(frame_tree_node->current_frame_host());
 }