cc: Introduce LayerContext

This introduces new LayerContext and LayerContextClient abstractions in
cc as well as their corresponding mojom interfaces in Viz.  These types
sit between a client-side LayerTreeHost and a corresponding
LayerTreeHostImpl which may reside in another process. They will be used
to experiment with GPU-side layer trees.

This CL only introduces minimal boilerplate and plumbing for a client to
construct a new client-side VizLayerContext over an established
CompositorFrameSink endpoint, to control a GPU-side LayerTreeHost owned
by that sink within Viz.

Test coverage and actual frame production from a LayerContext are done
in a follow-up CL.

Bug: 1431762
Change-Id: I86e9150aa034fe77300036816c8e2c5ae3999c55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4418567
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Auto-Submit: Ken Rockot <rockot@google.com>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1130132}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 05e11fe..d59e047 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -366,6 +366,8 @@
     "trees/latency_info_swap_promise.h",
     "trees/latency_info_swap_promise_monitor.cc",
     "trees/latency_info_swap_promise_monitor.h",
+    "trees/layer_context.h",
+    "trees/layer_context_client.h",
     "trees/layer_tree_frame_sink.cc",
     "trees/layer_tree_frame_sink.h",
     "trees/layer_tree_frame_sink_client.h",
@@ -381,6 +383,8 @@
     "trees/layer_tree_mutator.h",
     "trees/layer_tree_settings.cc",
     "trees/layer_tree_settings.h",
+    "trees/local_layer_context.cc",
+    "trees/local_layer_context.h",
     "trees/managed_memory_policy.cc",
     "trees/managed_memory_policy.h",
     "trees/mobile_optimized_viewport_util.cc",