chrome: Fix callbacks for different Profile types (see issue #2969)

- Only install network intercepts for Profiles that have an associated
  CefBrowserContext. For incognito windows the CefBrowserContext is
  associated with the OffTheRecordProfileImpl's original Profile.
- cefsimple: Return the default CefClient instance for browser windows
  created via the Chrome UI, and allow Chrome to show error pages.

tests/cefsimple/simple_handler.h

@@ -50,6 +50,9 @@ class SimpleHandler : public CefClient,
 
   bool IsClosing() const { return is_closing_; }
 
+  // Returns true if the Chrome runtime is enabled.
+  static bool IsChromeRuntimeEnabled();
+
  private:
   // Platform-specific implementation.
   void PlatformTitleChange(CefRefPtr<CefBrowser> browser,