[Offline Pages] Adds storage for kInvalidOfflineId.

Since kInvalidOfflineId is a static data member, it needs explicitly
defined storage as soon as it becomes 'odr-used'.  Replace a temporary
variable in RecentTabHelper with storage in offline_page_model.cc both
for clarity and to allow further references to kInvalidOfflineId to be
taken.

BUG=

Review-Url: https://codereview.chromium.org/2548033002
Cr-Commit-Position: refs/heads/master@{#436116}

chrome/browser/android/offline_pages/recent_tab_helper.cc

@@ -153,9 +153,8 @@ void RecentTabHelper::DidFinishNavigation(
   }
 
   if (offline_pages::IsOffliningRecentPagesEnabled()) {
-    int64_t proposed_id = OfflinePageModel::kInvalidOfflineId;
     download_info_ = base::MakeUnique<DownloadPageInfo>(
-        GetRecentPagesClientId(), proposed_id);
+        GetRecentPagesClientId(), OfflinePageModel::kInvalidOfflineId);
   } else {
     download_info_.reset();
   }

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) {
 }