ash: Add a test to cover AssociateTouchscreens with no internal display
ash: Add a test to cover AssociateTouchscreens with no internal display

A segfault was happening in AssociateTouchscreens if it found an
internal touchscreen but no internal display. This change introduces a
test to check that AssociateTouchscreens() will not crash when no
internal display is present when an internal touchscreen is identified.

BUG=515201
TEST=ash_unittests crashes before bug fix, and passes after bug fix
lands.
R=oshima@chromium.org

Review URL: https://codereview.chromium.org/1268683003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341344}

ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend