[Offline Pages] Fix msan test introduced by ClearStorageTask.

Fix msan test.

TBR=fgorski@chromium.org

Bug: NONE
Change-Id: I1a8083af308fc0869e0f20d22155d6a97f986f5e
Reviewed-on: https://chromium-review.googlesource.com/683601
Commit-Queue: Yafei Duan <romax@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504303}

components/offline_pages/core/model/offline_page_item_generator.h

@@ -34,7 +34,7 @@ class OfflinePageItemGenerator {
   std::string request_origin_;
   GURL url_;
   GURL original_url_;
-  int64_t file_size_;
+  int64_t file_size_ = 0;
   base::Time last_access_time_;
 };
 }  // namespace offline_pages