Signin for UserPolicy Signin/Signout Tests

Adds call to SigninManager::SignIn(...) as part of TestSuccessfulSignin
helper method. This is needed to simulate real signin and becomes
critical with upcoming changes* to FakeSigninManager.

*https://chromium-review.googlesource.com/c/chromium/src/+/1154985

Bug: 806781
Change-Id: I973859ef52bcddd978429dbb4ce9d913224c7b0d
Reviewed-on: https://chromium-review.googlesource.com/1160492
Commit-Queue: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580194}

chrome/browser/policy/cloud/user_policy_signin_service_unittest.cc

@@ -281,6 +281,9 @@ class UserPolicySigninServiceTest : public testing::Test {
     EXPECT_CALL(*this, OnPolicyRefresh(true)).Times(0);
     RegisterPolicyClientWithCallback(signin_service);
 
+    // Sign in to Chrome.
+    signin_manager_->SignIn(kTestGaiaId, kTestUser, "");
+
     // Mimic successful oauth token fetch.
     MakeOAuthTokenFetchSucceed();
 
@@ -433,7 +436,7 @@ TEST_F(UserPolicySigninServiceTest, InitRefreshTokenAvailableBeforeSignin) {
       profile_.get()->GetPrefs(), kTestGaiaId, kTestUser);
   GetTokenService()->UpdateCredentials(account_id, "oauth_login_refresh_token");
 
-  // Not ssigned in yet, so client registration should be deferred.
+  // Not signed in yet, so client registration should be deferred.
   ASSERT_FALSE(IsRequestActive());
 
   // Sign in to Chrome.