Disable default touch icons handling.

When no link rel=apple-touch-icon is present on a page, mobile chrome currently
sends requests for apple-touch-icon.png and apple-touch-icon-precomposed.png. A
very large majority of those requests fail with a 404. To save battery and data
plan, this CL disable those requests. Clients should use link rel=icon instead.

We can't turn of TOUCH_ICON_LOADING because we still want to support link
rel=apple-touch-icon

 Resubmitted after removing the test checking for this behavior in chromium: https://codereview.chromium.org/18903004/

R=thakis@chromium.org
BUG=258963,259681

Review URL: https://chromiumcodereview.appspot.com/18569004

git-svn-id: svn://svn.chromium.org/blink/trunk@154413 bbb929c8-8fbe-4397-9dbb-9b2b20218538