Discard protected tabs when all remaining tabs are protected

The protected tabs (tabs that CanDiscard() returns false) have lower
discard priority. When system is in low memory and all remaining tabs
are protected, some protected tab shall be discarded to avoid OOM.

Bug: 885087
Change-Id: I073a4946e82898a7f16c2ef11441e342c58b99e8
Reviewed-on: https://chromium-review.googlesource.com/c/1250823Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@chromium.org>
Commit-Queue: Vovo Yang <vovoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596085}

chrome/browser/resource_coordinator/tab_manager_delegate_chromeos.cc

@@ -536,9 +536,6 @@ bool TabManagerDelegate::KillArcProcess(const int nspid) {
 
 bool TabManagerDelegate::KillTab(LifecycleUnit* lifecycle_unit,
                                  ::mojom::LifecycleUnitDiscardReason reason) {
-  DecisionDetails decision_details;
-  if (!lifecycle_unit->CanDiscard(reason, &decision_details))
-    return false;
   bool did_discard = lifecycle_unit->Discard(reason);
   return did_discard;
 }