Call SetArcCpuRestriction() if StartArc() is called for opt-in

This ends up sending two D-Bus messages to session_manager, and
it is guaranteed that the message for starting ARC instance is
always sent first. This is because Chrome (with crrev.com/488879)
sends the D-Bus method directly in StartArc() without any thread
hops.

BUG=b:64327302
TEST=try, manually went through ARC opt-in

Change-Id: I97c4779b2ed5827b46559f9f7571a1090b4ced68
Reviewed-on: https://chromium-review.googlesource.com/601768
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491967}

chrome/browser/chromeos/arc/arc_session_manager.cc

@@ -871,6 +871,9 @@ void ArcSessionManager::OnAndroidManagementChecked(
           base::Bind(&ArcSessionManager::OnArcSignInTimeout,
                      weak_ptr_factory_.GetWeakPtr()));
       StartArc();
+      // Since opt-in is an explicit user (or admin) action, relax the
+      // cgroups restriction now.
+      SetArcCpuRestriction(false /* do_restrict */);
       break;
     case policy::AndroidManagementClient::Result::MANAGED:
       ShowArcSupportHostError(