[ios] Log UMA if didStartProvisionalNavigation doesn't add pending item.

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 7aabe25b46fa8d25f587af1601e3c41b2fee629c..5c731f047000244c0806a0563ef87a0898d75c45 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -5175,6 +5175,19 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
       [self registerLoadRequest:webViewURL];
     }
   }
+
+  if (![self currentSessionEntry]) {
+    // In this state CRWWebController will crash in |didCommitNavigation:|
+    // (crbug.com/676458). It's unclear if web controller could get into this
+    // state but it's one of the guesses for crbug.com/676458 root cause. Report
+    // UMA historgam if that happens.
+    // TODO(crbug.com/677552): Remove this historgam.
+    UMA_HISTOGRAM_BOOLEAN(
+        "WebController."
+        "StartProvisionalNavigationExitedWithEmptyNavigationManager",
+        true);
+  }
+
   // Ensure the URL is registered and loadPhase is as expected.
   DCHECK(_lastRegisteredRequestURL == webViewURL);
   DCHECK(self.loadPhase == web::LOAD_REQUESTED);