Refactor CryptAuth component to be more testable.

components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h

Index: components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
diff --git a/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h b/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
index 6287dfeabd21e7800072a735e72b2cf831eaa1d0..c5d58f3e910a92fec7f0636b792625291bbe5ec5 100644
--- a/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
+++ b/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
@@ -8,6 +8,9 @@
 #include <string>
 
 #include "base/callback_forward.h"
+#include "base/memory/scoped_ptr.h"
+
+class OAuth2TokenService;
 
 namespace proximity_auth {
 
@@ -18,6 +21,12 @@ class CryptAuthAccessTokenFetcher {
  public:
   virtual ~CryptAuthAccessTokenFetcher() {}
 
+  // Creates a default CryptAuthAccessTokenFetcher instance.
+  // |token_service| is not owned and should outlive the created instance.
+  static scoped_ptr<CryptAuthAccessTokenFetcher> CreateDefault(
+      OAuth2TokenService* token_service,
+      const std::string& account_id);
+
   // Fetches the access token asynchronously, invoking the callback upon
   // completion. If the fetch fails, the callback will be invoked with an empty
   // string.