fix: enable spell checking, which broke in upgrade to ch66

Chromium commit [03563dd163][1] changed the way that the
spellcheck-enabled status was checked, defaulting to false.

Added the first (!) test for spellchecking, too.

Fixes #13608.

[1]: 03563dd163

atom/renderer/api/atom_api_spell_check_client.h

@@ -46,6 +46,7 @@ class SpellCheckClient : public blink::WebSpellCheckPanelHostClient,
   void RequestCheckingOfText(
       const blink::WebString& textToCheck,
       blink::WebTextCheckingCompletion* completionCallback) override;
+  bool IsSpellCheckingEnabled() const override;
 
   // blink::WebSpellCheckPanelHostClient:
   void ShowSpellingUI(bool show) override;