SigninManager: Access prohibit_signout_ via accessor method

Minor cleanup to always access the information of whether signin is
prohibited via the accessor method rather than directly via the ivar.

Change-Id: I208930ce094d40d22a325cdca6fd36cf26da441a
Reviewed-on: https://chromium-review.googlesource.com/1142771Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576431}

components/signin/core/browser/fake_signin_manager.cc

@@ -75,7 +75,7 @@ void FakeSigninManager::SignIn(const std::string& gaia_id,
 }
 
 void FakeSigninManager::ForceSignOut() {
-  prohibit_signout_ = false;
+  ProhibitSignout(false);
   SignOut(signin_metrics::SIGNOUT_TEST,
           signin_metrics::SignoutDelete::IGNORE_METRIC);
 }

components/signin/core/browser/signin_manager.cc

@@ -214,7 +214,7 @@ void SigninManager::DoSignOut(
     return;
   }
 
-  if (prohibit_signout_) {
+  if (IsSignoutProhibited()) {
     DVLOG(1) << "Ignoring attempt to sign out while signout is prohibited";
     return;
   }