Add support for LayerTreeSettings (de)serialization

As part of serializing cc::LayerTreeHost, we also need to
serialize the cc::LayerTreeSettings.

BUG=561210
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1504273002

Cr-Commit-Position: refs/heads/master@{#364452}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 3be4ebb9..35551e62 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -884,6 +884,7 @@
     "trees/layer_tree_host_unittest_scroll.cc",
     "trees/layer_tree_host_unittest_video.cc",
     "trees/layer_tree_impl_unittest.cc",
+    "trees/layer_tree_settings_unittest.cc",
     "trees/occlusion_tracker_unittest.cc",
     "trees/occlusion_unittest.cc",
     "trees/property_tree_unittest.cc",