MacViews: Fix for multiple definitions of chrome::DragBookmarks.
There are 2 definitions of DragBookmarks, one for Cocoa and one for Views.
We want to link in one or the other, but they live in the same file as other
bookmark related utility functions. These functions are moved somewhere else
so that we can selectively link either bookmark_drag_drop_views.cc or
bookmark_drag_drop_cocoa.mm.
BUG=425229
Review URL: https://codereview.chromium.org/
814363002
Cr-Commit-Position: refs/heads/master@{#310318}