Improve documentation of LacrosChromeServiceImpl.

Change-Id: Ia074f4cf4f04ee755e41b938b229d4afb32f81de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416400
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808129}

chromeos/lacros/lacros_chrome_service_impl.h

@@ -66,6 +66,12 @@ class COMPONENT_EXPORT(CHROMEOS_LACROS) LacrosChromeServiceImpl {
   void BindReceiver(
       mojo::PendingReceiver<crosapi::mojom::LacrosChromeService> receiver);
 
+  // --------------------------------------------------------------------------
+  // mojo::Remote is sequence affine. The following methods are convenient
+  // helpers that expose pre-established Remotes that can only be used from the
+  // affine sequence (main thread).
+  // --------------------------------------------------------------------------
+
   // This must be called on the affine sequence.
   mojo::Remote<crosapi::mojom::MessageCenter>& message_center_remote() {
     DCHECK_CALLED_ON_VALID_SEQUENCE(affine_sequence_checker_);
@@ -93,6 +99,13 @@ class COMPONENT_EXPORT(CHROMEOS_LACROS) LacrosChromeServiceImpl {
     return hid_manager_remote_;
   }
 
+  // --------------------------------------------------------------------------
+  // Some clients will want to use mojo::Remotes on arbitrary sequences (e.g.
+  // background threads). The following methods allow the client to construct a
+  // mojo::Remote bound to an arbitrary sequence, and pass the other endpoint of
+  // the Remote (mojo::PendingReceiver) to ash to set up the interface.
+  // --------------------------------------------------------------------------
+
   // This may be called on any thread.
   void BindScreenManagerReceiver(
       mojo::PendingReceiver<crosapi::mojom::ScreenManager> pending_receiver);