Add basic framework for splitting thread proxy.

cc/BUILD.gn

Index: cc/BUILD.gn
diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 96fc142d49e462704ff050f2f7340a77ec78de8f..94fe90d299e585d4484809ccace0ffc77586c462 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -451,6 +451,8 @@ component("cc") {
     "tiles/tiling_set_raster_queue_required.h",
     "trees/blocking_task_runner.cc",
     "trees/blocking_task_runner.h",
+    "trees/channel_impl.h",
+    "trees/channel_main.h",
     "trees/damage_tracker.cc",
     "trees/damage_tracker.h",
     "trees/draw_property_utils.cc",
@@ -480,6 +482,8 @@ component("cc") {
     "trees/property_tree_builder.h",
     "trees/proxy.cc",
     "trees/proxy.h",
+    "trees/proxy_impl.h",
+    "trees/proxy_main.h",
     "trees/scoped_abort_remaining_swap_promises.h",
     "trees/single_thread_proxy.cc",
     "trees/single_thread_proxy.h",
@@ -487,6 +491,8 @@ component("cc") {
     "trees/swap_promise_monitor.h",
     "trees/thread_proxy.cc",
     "trees/thread_proxy.h",
+    "trees/threaded_channel.cc",
+    "trees/threaded_channel.h",
     "trees/tree_synchronizer.cc",
     "trees/tree_synchronizer.h",
   ]