[App Management] Add FakePageHandler

This CL adds a fake page handler to the app management page which is used when
accessing chrome://apps?fakeBackend=true.

Bug: 906508
Change-Id: Ib92a66a45b312efd14b5fb038d7cc506944dd716
Reviewed-on: https://chromium-review.googlesource.com/c/1360034
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614293}
6 files changed