bookmarks: Only try to get favicon for URL nodes.

Rearrange the logic to simplify it. And do it in terms of node's type.

Also, it seems better to only call BookmarkModel's GetFavicon function on
BookmarkNode URLs as we don't set favicon for folders.

TEST=open chrome, bookmark some pages, create some folders, drag them
in-and-out around, no regressions should be observed.

Review URL:

Cr-Commit-Position: refs/heads/master@{#307276}
2 files changed