Add underlays and split off common overlay functionality

Review URL: https://codereview.chromium.org/979693005

Cr-Commit-Position: refs/heads/master@{#323274}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 2d4a3933..ee83aece 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -248,8 +248,12 @@
     "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",