Fix crashers and issues when rendering blimp in tabs

blimp/client/core/contents/blimp_navigation_controller_impl.cc

Index: blimp/client/core/contents/blimp_navigation_controller_impl.cc
diff --git a/blimp/client/core/contents/blimp_navigation_controller_impl.cc b/blimp/client/core/contents/blimp_navigation_controller_impl.cc
index 7461e5db833b73c9d907507e30cf0f520928acc0..bd18ddd63deffd7130ed7aafa28d8b7355aaef05 100644
--- a/blimp/client/core/contents/blimp_navigation_controller_impl.cc
+++ b/blimp/client/core/contents/blimp_navigation_controller_impl.cc
@@ -23,7 +23,10 @@ BlimpNavigationControllerImpl::BlimpNavigationControllerImpl(
     navigation_feature_->SetDelegate(kDummyTabId, this);
 }
 
-BlimpNavigationControllerImpl::~BlimpNavigationControllerImpl() = default;
+BlimpNavigationControllerImpl::~BlimpNavigationControllerImpl() {
+  if (navigation_feature_)
+    navigation_feature_->RemoveDelegate(kDummyTabId);
+}
 
 void BlimpNavigationControllerImpl::LoadURL(const GURL& url) {
   current_url_ = url;