Updates comment for OnFrameSinkIdAllocated()

BUG=none
TEST=none (comment only change)

Change-Id: Icc0531dde3e613b6b87fd13620667659249324c1
Reviewed-on: https://chromium-review.googlesource.com/1097469Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567063}

services/ui/public/interfaces/window_tree.mojom

@@ -415,8 +415,9 @@ interface WindowTreeClient {
   // change.
   OnCaptureChanged(uint64 new_capture, uint64 old_capture);
 
-  // This is called after the client calls Embed() with |window|.
-  // |frame_sink_id| gives the new FrameSinkId of |window|.
+  // This is called on the client that initiated a call to Embed(), and informs
+  // the owner (embedder) of the new FrameSinkId of the window (embedding
+  // results in changing the FrameSinkId).
   OnFrameSinkIdAllocated(uint64 window, viz.mojom.FrameSinkId frame_sink_id);
 
   // Called in response to NewTopLevelWindow() successfully completing.