ash: Update window shadow assets

ash: Update window shadow assets

This version has a shorter bottom-edge shadow which makes the windows look
better when they are at the bottom of the screen near the launcher. It also
eliminates some subtle noise in the RGB channels.

BUG=168300
TEST=manual, shadows look correct on normal and high resolution displays

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=175855

[James Cook, 2013-01-09 17:14:21]

Oshima, can you approve this?  I can show you the assets in person if you would
like to verify they look OK.

[oshima, 2013-01-09 19:02:55]

lgtm