PlzNavigate: Properly call WebContents::DidStartNavigatingToPendingEntry

This CL makes it so that NavigatorImpl::Navigate will call
DidStartNavigatingToPendingEntry after creating the NaviagtionRequest when
PlzNavigate is enabled. This makes the test
NavigationControllerTest.ReloadOriginalRequestURL pass with PlzNavigate
enabled.

BUG=439423

Review URL: https://codereview.chromium.org/1035953002

Cr-Commit-Position: refs/heads/master@{#322543}

content/browser/frame_host/navigator_impl.cc

@@ -277,6 +277,11 @@ bool NavigatorImpl::NavigateToEntry(
     navigation_data_.reset(new NavigationMetricsData(
         navigation_start, entry.GetURL(), entry.restore_type()));
     RequestNavigation(frame_tree_node, entry, reload_type, navigation_start);
+
+    // Notify observers about navigation.
+    if (delegate_)
+      delegate_->DidStartNavigationToPendingEntry(entry.GetURL(), reload_type);
+
     return true;
   }