Adds a prefetching offline pages Feature, to gate future development.

BUG=701939

Review-Url: https://codereview.chromium.org/2741343013
Cr-Commit-Position: refs/heads/master@{#457942}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 1c01110..04be297 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1732,6 +1732,9 @@
     {"offline-pages-sharing", IDS_FLAGS_OFFLINE_PAGES_SHARING_NAME,
      IDS_FLAGS_OFFLINE_PAGES_SHARING_DESCRIPTION, kOsAndroid,
      FEATURE_VALUE_TYPE(offline_pages::kOfflinePagesSharingFeature)},
+    {"offline-pages-prefetching", IDS_FLAGS_OFFLINE_PAGES_PREFETCHING_NAME,
+     IDS_FLAGS_OFFLINE_PAGES_PREFETCHING_DESCRIPTION, kOsAndroid,
+     FEATURE_VALUE_TYPE(offline_pages::kPrefetchingOfflinePagesFeature)},
     {"background-loader-for-downloads",
      IDS_FLAGS_BACKGROUND_LOADER_FOR_DOWNLOADS_NAME,
      IDS_FLAGS_BACKGROUND_LOADER_FOR_DOWNLOADS_DESCRIPTION, kOsAndroid,