Set fromEntry to nil after discardNonCommittedItems.

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index 19a95a60e53e49fa95a2123711e9f9f759feb4a8..c695bf76f977afd25079aba11cdf90c0ca095d15 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -2164,6 +2164,14 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
       [self webWillFinishHistoryNavigationFromEntry:fromEntry];
       [self updateHTML5HistoryState];
     } else {
+      // TODO(crbug.com/691492): After discardNonCommittedItems, the item
+      // pointed by |fromEntry| may not be valid anymore.
+      // Set |fromEntry| to nil to avoid crash whan using the item.
+      // Remove when the fix is not needed anymore.
+      if ([fromEntry isEqual:sessionController.transientEntry] ||
+          [fromEntry isEqual:sessionController.pendingEntry]) {
+        fromEntry = nil;
+      }
       [sessionController discardNonCommittedItems];
       [sessionController setPendingItemIndex:index];
 
@@ -2401,7 +2409,10 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
 }
 
 - (void)webWillFinishHistoryNavigationFromEntry:(CRWSessionEntry*)fromEntry {
-  DCHECK(fromEntry);
+  // TODO(crbug.com/691492): Removing the DCHECK as |fromEntry| may be nil if
+  // the pending entry was discarded.
+  // Remove when the fix is not needed anymore.
+  // DCHECK(fromEntry);
   [self updateDesktopUserAgentForEntry:self.currentSessionEntry
                              fromEntry:fromEntry];
   [_delegate webWillFinishHistoryNavigationFromEntry:fromEntry];