Revert of Decrease the lag when switching between different categories in the Cros wallpaper selector.

Revert of Decrease the lag when switching between different categories in the Cros wallpaper selector. (patchset #5 id:140001 of https://codereview.chromium.org/1028513003/)

Reason for revert:
I suspect this is causing failures in the cros_trunk official builder:
http://master.chrome.corp.google.com:8011/builders/cros%20trunk/builds/31816

See https://code.google.com/p/chromium/issues/detail?id=470237

Original issue's description:
> Decrease the lag when switching between different categories in the Cros wallpaper selector.
>
> In the CL, we maintain a thumbnail list so that we don't need to fetch the thumbnail images from file system
> each time we switch between different categories. It might take some time at the first time, but will be
> instantly ready after that.
>
> And we also just preload grid items' thumbnail images in the current viewport, so that we don't experience noticeable
> lag when switching to a category at the first time.
>
> BUG=454932
> TEST=tested on devices (pixel & peppy)
>
> Committed: https://crrev.com/bb033ee318fc12de0b9e136c99acd25950b08e8f
> Cr-Commit-Position: refs/heads/master@{#322015}

TBR=bshe@chromium.org,xdai@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=454932
Committed: https://crrev.com/5d6b995d2344dc6246a902bbd98ed1c11c435d3f
Cr-Commit-Position: refs/heads/master@{#322063}

[James Cook, 2015-03-24 20:21:27]

Created Revert of Decrease the lag when switching between different categories
in the Cros wallpaper selector.