GitLab

Adds a flag to about:flags to make the "Add to Homescreen"
menu item visible.  Requires a downstream patch to actually put
the menu item in.

BUG=286156

Review URL: https://chromiumcodereview.appspot.com/23676005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221770 0039d316-1c4b-4281-b951-d872f2087c98