Resize webstore icons to a known size before placing them in the app list model.

Resize webstore icons to a known size before placing them in the app list model.

Icons from the webstore can be unusual sizes. Once installed,
ExtensionAppItem uses extension_misc::EXTENSION_ICON_MEDIUM (48) to load
it, so be consistent with that.

BUG=338253
TEST=(CrOS) Start installing a large app from the webstore (e.g. tank
riders). While it has a progress bar, drag it around the app launcher -
the icon should grow a little, not a lot (see bug for screenshots).

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284879

[calamity, 2014-07-23 03:50:34]

lgtm

[tapted, 2014-07-23 03:51:25]

The CQ bit was checked by tapted@chromium.org

[commit-bot: I haz the power, 2014-07-23 03:52:43]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tapted@chromium.org/400893002/20001

[commit-bot: I haz the power, 2014-07-23 04:56:07]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win8_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win8_chromium_rel/bui...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[commit-bot: I haz the power, 2014-07-23 07:05:47]

Change committed as 284879