Added BookmarkClient::CanBeEditedByUser.
CanBeEditedByUser indicates if a bookmark can be edited by the user or not.
All of the bookmarks in the bookmark_bar_node, the other_node and the
mobile_node are always editable but some of the extra nodes provided by the
client may be unmodifiable.
Also updated other methods to work only with user-editable nodes, and added
utility helpers to detect nodes that can or can't be edited.
BUG=49598
Review URL: https://codereview.chromium.org/
317333004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275523 0039d316-1c4b-4281-b951-d872f2087c98