Removes unnecessary subclassing of BookmarkCollectionView.

In the current code BookmarkCollectionView is only subclassed by
BookmarkFolderCollectionView. This hierarchy of classes is redundant
in the current scenario. The inheritance was introduced in legacy code,
when there were more than one subclass.

See go/bookmarks-refactor for more details.

BUG=705339

Review-Url: https://codereview.chromium.org/2921813002
Cr-Commit-Position: refs/heads/master@{#478941}
7 files changed