The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d2e20b8617a15152110d7e49baa003417785b206

commit d2e20b8617a15152110d7e49baa003417785b206
Author: Ria Jiang <riajiang@chromium.org>
Date: Wed Sep 06 19:51:21 2017

mus: Increment client_id for clients by 1.

Currently WindowServer has client_id 0, WindowManager 1, other clients
2++. This is confusing when we print out the id. Ids sent by clients in
WindowTree also have client_id 0 before we fill in the real one so it's
hard to tell if a window with client_id 0 is actually created by WS or
just haven't been processed (filled in the real id) by the WindowTree
yet.

This CL increments all client_id by 1, i.e. WS 1, WM 2, other clients
3++; and fixes related tests.

Bug:  761090 
Test: covered by tests
Change-Id: Ifd62d2c802c8fe99e6cddcda48b571d78924346e
Reviewed-on: https://chromium-review.googlesource.com/648255
Commit-Queue: Ria Jiang <riajiang@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500049}
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/ids.h
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/test_utils.cc
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/test_utils.h
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/window_manager_client_unittest.cc
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/window_manager_state_unittest.cc
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/window_server.cc
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/window_tree_client_unittest.cc
[modify] https://crrev.com/d2e20b8617a15152110d7e49baa003417785b206/services/ui/ws/window_tree_unittest.cc