Remove DCHECK from ArcBootPhaseThrottleObserver

DCHECK(arc_is_booting_) in ABPTO::OnArcInitialStart() was
consistently being hit in debug mode. This is because OnBootCompleted()
is actually called before OnArcInitialStart(), so arc_is_booting_ is
already set to false by the time OnArcInitialStart() is run.

Bug: b:133417350
Test: ABPTO unittests
Change-Id: I023ee2841e7ed986b68bab06df1a57d1ac6ac8be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872415
Commit-Queue: Willie Koomson <wvk@google.com>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Auto-Submit: Willie Koomson <wvk@google.com>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707969}

chrome/browser/chromeos/arc/instance_throttle/arc_boot_phase_throttle_observer.cc

@@ -51,7 +51,6 @@ void ArcBootPhaseThrottleObserver::OnArcStarted() {
 }
 
 void ArcBootPhaseThrottleObserver::OnArcInitialStart() {
-  DCHECK(arc_is_booting_);
   arc_is_booting_ = false;
   MaybeSetActive();
 }