Moved NavigationManagerImpl serialization out of CRWSessionController.

ios/web/navigation/navigation_manager_impl.mm

Index: ios/web/navigation/navigation_manager_impl.mm
diff --git a/ios/web/navigation/navigation_manager_impl.mm b/ios/web/navigation/navigation_manager_impl.mm
index 6d7835c92d4c47ab694a11b77e8198b34d45afa2..31a3377b3ade1cfd36a29384fade3e5df96e02c6 100644
--- a/ios/web/navigation/navigation_manager_impl.mm
+++ b/ios/web/navigation/navigation_manager_impl.mm
@@ -70,14 +70,8 @@ bool AreURLsInPageNavigation(const GURL& existing_url, const GURL& new_url) {
   return *this;
 }
 
-NavigationManagerImpl::NavigationManagerImpl(
-    NavigationManagerDelegate* delegate,
-    BrowserState* browser_state)
-    : delegate_(delegate),
-      browser_state_(browser_state),
-      facade_delegate_(nullptr) {
-  DCHECK(browser_state_);
-}
+NavigationManagerImpl::NavigationManagerImpl()
+    : delegate_(nullptr), browser_state_(nullptr), facade_delegate_(nullptr) {}
 
 NavigationManagerImpl::~NavigationManagerImpl() {
   // The facade layer should be deleted before this object.
@@ -86,6 +80,14 @@ bool AreURLsInPageNavigation(const GURL& existing_url, const GURL& new_url) {
   [session_controller_ setNavigationManager:nullptr];
 }
 
+void NavigationManagerImpl::SetDelegate(NavigationManagerDelegate* delegate) {
+  delegate_ = delegate;
+}
+
+void NavigationManagerImpl::SetBrowserState(BrowserState* browser_state) {
+  browser_state_ = browser_state;
+}
+
 void NavigationManagerImpl::SetSessionController(
     CRWSessionController* session_controller) {
   session_controller_.reset(session_controller);