Added support for (de)serializing cc::LayerTreeDebugState

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

This CL focuses only on the debug state.

BUG=561210

CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1469393004

Cr-Commit-Position: refs/heads/master@{#362534}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 60ac70c..67981ef 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -755,6 +755,7 @@
     "base/tiling_data_unittest.cc",
     "base/unique_notifier_unittest.cc",
     "debug/frame_timing_tracker_unittest.cc",
+    "debug/layer_tree_debug_state_unittest.cc",
     "debug/micro_benchmark_controller_unittest.cc",
     "debug/rendering_stats_unittest.cc",
     "input/scroll_state_unittest.cc",