Generate container icons for generated streamlined hosted apps.

When generating apps from web pages, rather than stretching small favicons for
use on the taskbar, chromeos shelf, or app launcher, generate an icon which is
a rounded rect with a strip of the app icon's dominant color and lay the 1x
icon in the centre of this container.

See bug for screenshots.

BUG=315632,318607

Review URL: https://codereview.chromium.org/140693008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245973 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245973