[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}
(cherry picked from commit 34b66e5f77f6efa3708efda9d0fc099881b1fedf)

Review URL: https://codereview.chromium.org/1484983002 .

Cr-Commit-Position: refs/branch-heads/2564@{#167}
Cr-Branched-From: 1283eca15bd9f772387f75241576cde7bdec7f54-refs/heads/master@{#359700}
3 files changed