Do not create pending NavigationEntry when a transient one is present.

content/browser/frame_host/navigator_impl.cc

Index: content/browser/frame_host/navigator_impl.cc
diff --git a/content/browser/frame_host/navigator_impl.cc b/content/browser/frame_host/navigator_impl.cc
index 6afb53557d27cc587d113398d3a48b0f2072e129..ec7f4013f4a1c2b305ba9287b10b1abeb328397d 100644
--- a/content/browser/frame_host/navigator_impl.cc
+++ b/content/browser/frame_host/navigator_impl.cc
@@ -1025,7 +1025,12 @@ void NavigatorImpl::DidStartMainFrameNavigation(
   NavigationEntryImpl* pending_entry = controller_->GetPendingEntry();
   bool has_browser_initiated_pending_entry =
       pending_entry && !pending_entry->is_renderer_initiated();
-  if (!has_browser_initiated_pending_entry) {
+
+  // If there is a transient entry, creating a new pending entry will result
+  // in deleting it, which leads to inconsistent state.
+  bool has_transient_entry = !!controller_->GetTransientEntry();

[Charlie Reis, 2016/04/13 06:03:37]

I still think this is correct, but I just want to make sure we're doing the
right thing.

Suppose URL A causes an interstitial in a new tab, and it has a
renderer-initiated pending entry.  Then renderer-initiated URL B gets here.  We
skip the block below because there's an interstitial, so we still have the
pending entry for A.  If the interstitial gets dismissed, the renderer-initiated
pending entry could be displayed, and it would be for A rather than B?  (Or
maybe we'd get a DidFail and cancel it?)

I'm mainly wondering if we need to discard the pending entry (and not the
transient entry) in this case, or if just skipping the block below is
sufficient.

[Charlie Reis, 2016/04/14 23:02:45]

From your email, it sounds like we'll hang if we proceed after trying to
navigate to B.  I'm not too concerned, then.

+
+  if (!has_browser_initiated_pending_entry && !has_transient_entry) {
     std::unique_ptr<NavigationEntryImpl> entry =
         NavigationEntryImpl::FromNavigationEntry(
             controller_->CreateNavigationEntry(