bookmarks: Only try to get favicon for URL nodes.

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.

BUG=None
TEST=open chrome, bookmark some pages, create some folders, drag them
in-and-out around, no regressions should be observed.
R=sky@chromium.org

Committed: https://crrev.com/c0baf0baa80a7881f82d89ad8f5b917ef49598ec
Cr-Commit-Position: refs/heads/master@{#307276}

[tfarina, 2014-12-08 01:10:13]

Patchset #1 (id:1) has been deleted

[sky, 2014-12-08 15:56:58]

LGTM

[tfarina, 2014-12-08 17:14:14]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2014-12-08 17:15:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/763903003/20001

[commit-bot: I haz the power, 2014-12-08 18:01:29]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2014-12-08 18:02:40]

Patchset 1 (id:??) landed as
https://crrev.com/c0baf0baa80a7881f82d89ad8f5b917ef49598ec
Cr-Commit-Position: refs/heads/master@{#307276}