Add underlays and split off common overlay functionality

cc/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
 component("cc") {
   sources = [
     "animation/animation.cc",
     "animation/animation.h",
@@ skipping 259 matching lines @@
     "output/managed_memory_policy.cc",
     "output/managed_memory_policy.h",
     "output/output_surface.cc",
     "output/output_surface.h",
     "output/output_surface_client.h",
     "output/overlay_candidate.cc",
     "output/overlay_candidate.h",
     "output/overlay_candidate_validator.h",
     "output/overlay_processor.cc",
     "output/overlay_processor.h",
+    "output/overlay_strategy_common.cc",
+    "output/overlay_strategy_common.h",
     "output/overlay_strategy_single_on_top.cc",
     "output/overlay_strategy_single_on_top.h",
+    "output/overlay_strategy_underlay.cc",
+    "output/overlay_strategy_underlay.h",
     "output/program_binding.cc",
     "output/program_binding.h",
     "output/render_surface_filters.cc",
     "output/render_surface_filters.h",
     "output/renderer.cc",
     "output/renderer.h",
     "output/renderer_settings.cc",
     "output/renderer_settings.h",
     "output/shader.cc",
     "output/shader.h",
@@ skipping 631 matching lines @@
     "//testing/gmock",
     "//testing/gtest",
     "//testing/perf",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",
   ]
 }
 # When adding support for isolates, please have a look at run-time dependencies
 # in the cc_unittests_run target in cc_tests.gyp.