Reading List: Mark entry read when opening external app.

ios/chrome/browser/reading_list/reading_list_web_state_observer.mm

Index: ios/chrome/browser/reading_list/reading_list_web_state_observer.mm
diff --git a/ios/chrome/browser/reading_list/reading_list_web_state_observer.mm b/ios/chrome/browser/reading_list/reading_list_web_state_observer.mm
index 0ca54adae018555d354449622a0646e3f43d9308..500763db3bde4969514a9fc0c3b6205a2769000f 100644
--- a/ios/chrome/browser/reading_list/reading_list_web_state_observer.mm
+++ b/ios/chrome/browser/reading_list/reading_list_web_state_observer.mm
@@ -327,3 +327,15 @@ void ReadingListWebStateObserver::LoadOfflineReadingListEntry() {
   reading_list_model_->SetReadStatus(entry->URL(), true);
   UMA_HISTOGRAM_BOOLEAN("ReadingList.OfflineVersionDisplayed", true);
 }
+
+void ReadingListWebStateObserver::CurrentNavigationWillContinueInExternalApp() {
+  if (!pending_url_.is_valid()) {
+    return;
+  }
+  const ReadingListEntry* entry =
+      reading_list_model_->GetEntryByURL(pending_url_);
+  if (!entry) {
+    return;
+  }
+  reading_list_model_->SetReadStatus(pending_url_, true);
+}