app_list ExtensionAppModelBuilder: Observe and handle changes to prefs::kHideWebStoreIcon.
The pref was already implicitly checked on initialization during PopulateApps (via extensions::ui_util::ShouldDisplayInAppLauncher).
This CL adds handling of pref changes by re-checking ShouldDisplayInAppLauncher. It also extends an existing test to cover these cases.
Handling of prefs::kHideWebStoreIcon was originally added in https://codereview.chromium.org/
12038067
BUG=
Review URL: https://codereview.chromium.org/
409123002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285886 0039d316-1c4b-4281-b951-d872f2087c98