Don't crash when closing interstitial pages.

content/browser/frame_host/interstitial_page_impl.cc

Index: content/browser/frame_host/interstitial_page_impl.cc
diff --git a/content/browser/frame_host/interstitial_page_impl.cc b/content/browser/frame_host/interstitial_page_impl.cc
index 10f99b4eadc3586208c6768500a0ec1ef3fdfff2..3d9240833811380b24e0fb1701c030faa2d97541 100644
--- a/content/browser/frame_host/interstitial_page_impl.cc
+++ b/content/browser/frame_host/interstitial_page_impl.cc
@@ -357,7 +357,18 @@ void InterstitialPageImpl::NavigationEntryCommitted(
   OnNavigatingAwayOrTabClosing();
 }
 
+void InterstitialPageImpl::WebContentsWillBeDestroyed() {
+  OnNavigatingAwayOrTabClosing();
+}
+
 void InterstitialPageImpl::WebContentsDestroyed(WebContents* web_contents) {
+  // WebContentsImpl will only call WebContentsWillBeDestroyed for interstitial
+  // pages that it knows about, pages that called
+  // WebContentsImpl::AttachInterstitialPage. But it's possible to have an
+  // interstitial page that never progressed that far. In that case, ensure that
+  // this interstitial page is destroyed. (And if it was attached, and
+  // OnNavigatingAwayOrTabClosing was called, it's safe to call
+  // OnNavigatingAwayOrTabClosing twice.)
   OnNavigatingAwayOrTabClosing();
 }