[Offline pages] Making sure visit button works on tablet

Per suggestion from kkimlabs@ this patch:
* Moves a piece of logic responsible for opening bookmarks from
  EnhancedBookmarkManager to EnhancedBookmarkUtils
* Calls EnhancedBookmarkUtils#openBookmark from EnhancedBookmarkEditActivity
  when handling the Visit button.

BUG=560133
R=kkimlabs@chromium.org

Review URL: https://codereview.chromium.org/1475513008

Cr-Commit-Position: refs/heads/master@{#361809}
3 files changed