cros: Fix ash ScreenshotDelegate ownership and creation

Many ash_unittests fail under --mus and --mash because the
TestScreenshotDelegate is not created. This happens because of some
legacy AcceleratorControllerDelegate code that dates back ~18 months
to when we couldn't use aura::Window* in mash code. That isn't a
problem anymore.

* Move ownership of ScreenshotDelegate to ScreenshotController
* Create the delegate in ShellDelegate, similar to how we do other
  ash delegates
* Move screenshot accelerator code back into AcceleratorController
* Re-enable tests under mus and mash via filter files

Screenshots don't work yet under --mus and --mash due to lack of
graphics readback support and lack of a mojo interface, but we now
exercise more of the UI and accelerator handling code.

Bug: 557397, 632111
Test: ash_unittests (--mus, --mash), manually take screenshots
Change-Id: I7dd1946ddc30bddb407c9dbf6095caf8efb4fd1f
Reviewed-on: https://chromium-review.googlesource.com/744295
Reviewed-by: Michael Wasserman <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Vladislav Kaznacheev <[email protected]>
Commit-Queue: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/master@{#513018}
34 files changed