Add a comment and DCHECK to require |session_storage| to be non-nil in

WebState::CreateWithStorageSession().

Because it looks passing nil doesn't work. Alternatively I can update
the code to make CreateWithStorageSession(params, nil) to be equivalent
of Create(params) if you prefer.

Background:
I wrote code to expect that CreateWithStorageSession(params, nil) is
equivalent of Create(params), resulted in a weird error.

Review-Url: https://codereview.chromium.org/2802613005
Cr-Commit-Position: refs/heads/master@{#462766}

ios/web/public/web_state/web_state.h

@@ -93,7 +93,8 @@ class WebState : public base::SupportsUserData {
   // Creates a new WebState.
   static std::unique_ptr<WebState> Create(const CreateParams& params);
 
-  // Creates a new WebState from a serialized NavigationManager.
+  // Creates a new WebState from a serialized representation of the session.
+  // |session_storage| must not be nil.
   static std::unique_ptr<WebState> CreateWithStorageSession(
       const CreateParams& params,
       CRWSessionStorage* session_storage);

ios/web/web_state/web_state_impl.mm

@@ -57,6 +57,7 @@ std::unique_ptr<WebState> WebState::Create(const CreateParams& params) {
 std::unique_ptr<WebState> WebState::CreateWithStorageSession(
     const CreateParams& params,
     CRWSessionStorage* session_storage) {
+  DCHECK(session_storage);
   std::unique_ptr<WebStateImpl> web_state(
       new WebStateImpl(params, session_storage));