wrongbaseurl OSX App List Pager and root view controller.

OSX App List Pager and root view controller.

Adds a root AppListView for the OSX app list, and moves ownership of
the AppListViewDelegate there (as well as principal ownership of the
GridController). The root view contains the apps grid, a pager control,
and a background. An AppsPaginationModelObserver feeds scroll changes
on the grid to the view controller to update the pager (an
NSSegmentedControl).

BUG=138633
TEST=Added app_list_unittests: AppListViewControllerTest.*, AppsGridControllerTest.PaginationObserver*