Fix a race between navigation and an interstitial.

content/browser/frame_host/navigation_controller_impl.cc

Index: content/browser/frame_host/navigation_controller_impl.cc
diff --git a/content/browser/frame_host/navigation_controller_impl.cc b/content/browser/frame_host/navigation_controller_impl.cc
index 1f07ec664e0b2f0f212e746e2fb5d89d9d80db42..3ece628e668dc4daff0f6a4af65d659d5403cadb 100644
--- a/content/browser/frame_host/navigation_controller_impl.cc
+++ b/content/browser/frame_host/navigation_controller_impl.cc
@@ -1880,10 +1880,15 @@ void NavigationControllerImpl::NavigateToPendingEntry(ReloadType reload_type) {
       pending_entry_->GetTransitionType() & ui::PAGE_TRANSITION_FORWARD_BACK) {
     delegate_->Stop();
 
-    // If an interstitial page is showing, we want to close it to get back
-    // to what was showing before.
-    if (delegate_->GetInterstitialPage())
-      delegate_->GetInterstitialPage()->DontProceed();
+    // If an interstitial page is showing, we want to close it to get back to
+    // what was showing before. There are two ways to get the interstitial page
+    // given a WebContents, but WebContents::GetInterstitialPage might return
+    // null if the interstitial was shown but not yet visible while

[Charlie Reis, 2017/06/22 16:19:38]

nit: s/shown/created/, perhaps?

[Avi (use Gerrit), 2017/06/22 18:08:51]

I'm talking about the range in which WebContents::GetInterstitialPage and
InterstitialPage::GetInterstitialPage differ. It's precisely the time from when
the interstitial was told to "show" through the time the interstitial is visible
that GetInterstitialPage returns null but GetInterstitialPage returns the
interstitial.

[Charlie Reis, 2017/06/22 21:06:00]

Ok.  Can you rephrase to say "if the interstitial's Show() method was called but
it is not yet visible, while..."?  I'm finding "shown but not visible" hard to
understand without clarifying that one of them is a method.

[Avi (use Gerrit), 2017/06/22 21:46:42]

I rewrote it a few times, and I'm now much happier with the phrasing. Thank you
for the push.

+    // InterstitialPage::GetInterstitialPage will return such an interstitial.
+    InterstitialPage* interstitial =
+        InterstitialPage::GetInterstitialPage(GetWebContents());
+    if (interstitial)
+      interstitial->DontProceed();
 
     DiscardNonCommittedEntries();
     return;