[ios] Temporary fix for CRWSessionStorage being released early in ClosingWebStateObserver.

RestoreIOSLiveTab is not compiled with ARC and therefore is not properly
retaining the CRWSessionStorage* which it is passed. Temporarily work
around this bug by keeping a strong pointer to the CRWSessionStorage* in
ClosingWebStateObserver, until RestoreIOSLiveTab can be fixed properly.

BUG=1070852

Change-Id: Ic3d8a4d0cc3c29bfae4f657de06b98aa869b3bc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150035Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759116}

ios/chrome/browser/tabs/closing_web_state_observer.mm

@@ -67,8 +67,13 @@
 
   web::NavigationManager* navigationManager = webState->GetNavigationManager();
   if (navigationManager->IsRestoreSessionInProgress()) {
-    auto live_tab = std::make_unique<sessions::RestoreIOSLiveTab>(
-        webState->BuildSessionStorage());
+    // TODO(crbug.com/1070852): RestoreIOSLiveTab is not compiled with ARC and
+    // therefore is not properly retaining the CRWSessionStorage* which it is
+    // passed. Temporarily work around this bug by keeping a strong pointer to
+    // the CRWSessionStorage* here, until RestoreIOSLiveTab can be fixed
+    // properly.
+    CRWSessionStorage* storage = webState->BuildSessionStorage();
+    auto live_tab = std::make_unique<sessions::RestoreIOSLiveTab>(storage);
     _restoreService->CreateHistoricalTab(live_tab.get(), atIndex);
     return;
   }