[Views] Move modality tests from widget_unittest.cc to desktop_native_widget_aura_unittest.cc.

These tests were both under USE_AURA when they were originally
added (r226609 and r227041):
WindowMouseModalityTest
WindowModalityActivationTest

Later, Aura became implied with Views (r255976).

With MacViews, we could if-def these out again, but since they're
also if-def'd out on OS_CHROMEOS, move them to
desktop_native_widget_aura_unittest.cc.

This also moves TestDesktopWidgetDelegate to widget_test.h and
EventCountView to test_views.h.

BUG=378134

Review URL: https://codereview.chromium.org/1254963010

Cr-Commit-Position: refs/heads/master@{#341638}