UI: Move Shadow from wm::Shadow to ui::Shadow

To implement the Material Refresh UI, we need to draw this
elevation-based shadow on all platforms, not just ones that we use
the Window manager on.

Therefore we are moving this Shadow class from ui/wm to ui/compositor.

Forked off of:
https://chromium-review.googlesource.com/c/chromium/src/+/869699.

Bug: 823535
Change-Id: Ib12ca299ab15512022228283d946c8403751525a
Reviewed-on: https://chromium-review.googlesource.com/978590
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546923}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index c75b96e..2f74c3a9 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -359,6 +359,7 @@
 
 test("aura_unittests") {
   sources = [
+    "../compositor_extra/shadow_unittest.cc",
     "gestures/gesture_recognizer_unittest.cc",
     "hit_test_data_provider_aura_unittest.cc",
     "mus/drag_drop_controller_mus_unittest.cc",
@@ -393,6 +394,7 @@
     "//testing/gtest",
     "//ui/base:test_support",
     "//ui/compositor:test_support",
+    "//ui/compositor_extra",
     "//ui/display:test_support",
     "//ui/events:gesture_detection",
     "//ui/events:test_support",