Fix a crash in -[CWVWebView updateWebStateVisibility].

The app crashes if the method is called after |_webState| is deallocated.

Change-Id: I8c06cddc8ea3a39fd2c8c5d05d4c17fa0bd071d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830251Reviewed-by: John Wu <jzw@chromium.org>
Commit-Queue: Hiroshi Ichikawa <ichikawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701017}

ios/web_view/internal/cwv_web_view.mm

@@ -592,6 +592,9 @@ static NSString* gUserAgentProduct = nil;
 #pragma mark - Private methods
 
 - (void)updateWebStateVisibility {
+  if (_webState == nullptr) {
+    return;
+  }
   if (self.superview) {
     _webState->WasShown();
   } else {