Compiles //components/sessions/core/ios with ARC.

These files were inadvertently missing ARC guards and were not being
compiled with ARC.

BUG=1070852,1070933

Change-Id: I1005d41c26233d858aaee3670d3ce8b69ab561a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150300
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759257}

components/sessions/BUILD.gn

@@ -55,6 +55,7 @@ if (!is_ios) {
   }
 } else {
   source_set("sessions") {
+    configs += [ "//build/config/compiler:enable_arc" ]
     sources = [
       "ios/ios_live_tab.h",
       "ios/ios_live_tab.mm",

components/sessions/ios/ios_live_tab.mm

@@ -6,6 +6,10 @@
 #include "base/memory/ptr_util.h"
 #include "ios/web/public/navigation/navigation_manager.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 const char kIOSLiveTabWebStateUserDataKey[] = "ios_live_tab";
 }

components/sessions/ios/ios_restore_live_tab.mm

@@ -8,6 +8,10 @@
 #include "ios/web/public/session/crw_navigation_item_storage.h"
 #include "ios/web/public/session/crw_session_storage.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace sessions {
 
 RestoreIOSLiveTab::RestoreIOSLiveTab(CRWSessionStorage* session)

components/sessions/ios/ios_serialized_navigation_builder.mm

@@ -10,6 +10,10 @@
 #include "ios/web/public/navigation/referrer.h"
 #include "ios/web/public/session/crw_navigation_item_storage.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace sessions {
 
 // static