Fix unique_ptr usage in handling OAuth2TokenService::StartRequest result

Calling release() on a temporary unique_ptr to reset another unique_ptr with
resulting raw pointer is verbose and obscure. It also loses custom deleter
(see C++11 standard, clause 20.7.1.2.3.1).

BUG=none

Review-Url: https://codereview.chromium.org/2755403002
Cr-Original-Commit-Position: refs/heads/master@{#458047}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e32318796eaf782ee8afe8ec047b1288f756f644
diff --git a/gaia/oauth2_token_service_unittest.cc b/gaia/oauth2_token_service_unittest.cc
index bd770cf..b233d89 100644
--- a/gaia/oauth2_token_service_unittest.cc
+++ b/gaia/oauth2_token_service_unittest.cc
@@ -36,8 +36,8 @@
   void OnGetTokenFailure(const OAuth2TokenService::Request* request,
                          const GoogleServiceAuthError& error) override {
     TestingOAuth2TokenServiceConsumer::OnGetTokenFailure(request, error);
-    request_.reset(oauth2_service_->StartRequest(
-        account_id_, OAuth2TokenService::ScopeSet(), this).release());
+    request_ = oauth2_service_->StartRequest(
+        account_id_, OAuth2TokenService::ScopeSet(), this);
   }
 
   OAuth2TokenService* oauth2_service_;