GitLab

This CL does:
1. Url index uses flat_set instead of set.
2. Do not load image data and page data from bookmarks metadata into
   memory these are deprecated.
3. BookmarkNode stores ptr to icon_url instead of GURL

BUG=691698

Review-Url: https://codereview.chromium.org/2883523002
Cr-Commit-Position: refs/heads/master@{#476158}