RC: Do not allow freezing a tab that is discarded.

In the current state, TabManager::PerformStateTransitions() needlessly
tries to freeze discarded tabs.

Bug: 775644
Change-Id: Ieb26cd4a1251b8575308122311bdeabcff831acd
Reviewed-on: https://chromium-review.googlesource.com/1093310Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565777}

chrome/browser/resource_coordinator/tab_lifecycle_unit.cc

@@ -258,9 +258,11 @@ bool TabLifecycleUnitSource::TabLifecycleUnit::CanFreeze(
   // rejection reasons. These aren't worth reporting about, as they have nothing
   // to do with the content itself.
 
-  // Can't freeze a tab that is already frozen.
-  if (IsFrozenOrPendingFreeze(GetState()))
+  // Can't freeze a tab that is already frozen or discarded.
+  if (IsFrozenOrPendingFreeze(GetState()) ||
+      IsDiscardedOrPendingDiscard(GetState())) {
     return false;
+  }
 
   // Allow a page to load fully before freezing it.
   if (TabLoadTracker::Get()->GetLoadingState(GetWebContents()) !=