Update comment describing parent_id

The semantics of this property change pre- and post- surface synchronization.

BUG=789259

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I511d0c6fe2c465c7ef36076bd3b461f7d216a081
Reviewed-on: https://chromium-review.googlesource.com/808997Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522137}

services/viz/public/interfaces/compositing/local_surface_id.mojom

@@ -7,8 +7,9 @@ module viz.mojom;
 import "mojo/common/unguessable_token.mojom";
 
 struct LocalSurfaceId {
-  // An identifier allocated by the client uniquely identifying a surface within
-  // a client process.
+  // An identifier to uniquely identify a surface.
+  // Prior to surface synchronization, this is allocated by the child.
+  // With surface synchronization enabled, this is allocated by the parent.
   uint32 parent_id;
 
   mojo.common.mojom.UnguessableToken nonce;