Enable copresencePrivate API in incognito mode

This CL fixes a crash on opening chrome://copresence
in incognito window.

BUG=485992

Review URL: https://codereview.chromium.org/1134733002

Cr-Commit-Position: refs/heads/master@{#329447}

chrome/browser/extensions/api/copresence_private/copresence_private_api.h

@@ -41,6 +41,7 @@ class CopresencePrivateService final : public BrowserContextKeyedAPI {
   friend class BrowserContextKeyedAPIFactory<CopresencePrivateService>;
 
   // BrowserContextKeyedAPI implementation.
+  static const bool kServiceRedirectedInIncognito = true;
   static const char* service_name() { return "CopresencePrivateService"; }
 
   bool initialized_;