Improve GaiaAuthFetcher's API for its subclasses.

As |GaiaAuthFetcher::CreateAndStartGaiaFetcher| can already be
specialized by a subclass, |GaiaAuthFetcher::CancelRequest| should as
well, to allow specialized fetchers to be cancelled.

BUG=495597

Review URL: https://codereview.chromium.org/1153293004

Cr-Commit-Position: refs/heads/master@{#333045}

google_apis/gaia/gaia_auth_fetcher.cc

@@ -224,6 +224,10 @@ bool GaiaAuthFetcher::HasPendingFetch() {
   return fetch_pending_;
 }
 
+void GaiaAuthFetcher::SetPendingFetch(bool pending_fetch) {
+  fetch_pending_ = pending_fetch;
+}
+
 void GaiaAuthFetcher::CancelRequest() {
   fetcher_.reset();
   fetch_pending_ = false;

google_apis/gaia/gaia_auth_fetcher.h

@@ -224,7 +224,7 @@ class GaiaAuthFetcher : public net::URLFetcherDelegate {
   bool HasPendingFetch();
 
   // Stop any URL fetches in progress.
-  void CancelRequest();
+  virtual void CancelRequest();
 
   // From a URLFetcher result, generate an appropriate error.
   // From the API documentation, both IssueAuthToken and ClientLogin have
@@ -255,6 +255,8 @@ class GaiaAuthFetcher : public net::URLFetcherDelegate {
                               const net::URLRequestStatus& status,
                               int response_code);
 
+  void SetPendingFetch(bool pending_fetch);
+
  private:
   // ClientLogin body constants that don't change
   static const char kCookiePersistence[];