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}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index aac62209..b02ca41a 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -247,6 +247,8 @@
     "test/aura_test_suite_setup.h",
     "test/aura_test_utils.cc",
     "test/aura_test_utils.h",
+    "test/default_event_generator_delegate.cc",
+    "test/default_event_generator_delegate.h",
     "test/env_test_helper.cc",
     "test/env_test_helper.h",
     "test/event_generator_delegate_aura.cc",