arc: Check non-null profile in ArcSessionManager::SetProfile()

Currently we only call SetProfile once during initialization with
non-null |profile|. Remove preconditions that allow null |profile| if
|profile_| is already set.

This simplifies metrics recording logic in upcoming change as we no
longer consider null |profile|.

Bug: 929583
Change-Id: I68b4ac064ec07984844dc04f92ea46cf36613a21
Reviewed-on: https://chromium-review.googlesource.com/c/1493632Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Shao-Chuan Lee <shaochuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636816}

chrome/browser/chromeos/arc/arc_session_manager.cc

@@ -435,8 +435,8 @@ bool ArcSessionManager::IsAllowed() const {
 
 void ArcSessionManager::SetProfile(Profile* profile) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
-  DCHECK(!profile || !profile_);
-  DCHECK(!profile || IsArcAllowedForProfile(profile));
+  DCHECK(!profile_);
+  DCHECK(IsArcAllowedForProfile(profile));
   profile_ = profile;
   UpdatePersistentUMAState();
 }