[Offline Pages] Adds storage for kInvalidOfflineId.

components/offline_pages/offline_page_model.cc

Index: components/offline_pages/offline_page_model.cc
diff --git a/components/offline_pages/offline_page_model.cc b/components/offline_pages/offline_page_model.cc
index 24b4dd097f2cf8d415e88bd94d8177629ab3788f..39cd918dc744d35c3daaa4997432072ca2ef50d3 100644
--- a/components/offline_pages/offline_page_model.cc
+++ b/components/offline_pages/offline_page_model.cc
@@ -8,6 +8,8 @@
 
 namespace offline_pages {
 
+const int64_t OfflinePageModel::kInvalidOfflineId;
+
 OfflinePageModel::SavePageParams::SavePageParams()
     : proposed_offline_id(OfflinePageModel::kInvalidOfflineId) {
 }