Share common code in event generator delegates where possible.

This factors DefaultEventGeneratorDelegate out to a separate file and uses it in
chromecast/.  It also adds a default ScreenPositionClient getter to
EventGeneratorDelegateAura and removes the duplicate overrides in various
subclasses.

Bug: none
Change-Id: I449cf1c10daa4a23b197679dcb428acbbe54a52f
Reviewed-on: https://chromium-review.googlesource.com/c/1338577
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609094}

• ash/test/ash_test_base.cc[diff]
• chrome/test/base/default_ash_event_generator_delegate.cc[diff]
• chromecast/graphics/accessibility/partial_magnification_controller_unittest.cc[diff]
• chromecast/graphics/gestures/side_swipe_detector_test.cc[diff]
• services/ws/window_service_test_setup.cc[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/test/default_event_generator_delegate.cc[Added - diff]
• ui/aura/test/default_event_generator_delegate.h[Added - diff]
• ui/aura/test/event_generator_delegate_aura.cc[diff]
• ui/aura/test/event_generator_delegate_aura.h[diff]