So, my original understanding was wrong in the sense that the GpuClient for a browser child process uses the same client id as its corresponding ChildProcessHostImpl, so it cannot be generated by the GpuClient itself.

Short term, we can still pass in the id to the GpuClient and make GpuClient::Create() function a standalone factory function, so that Ash clients continue to use the same id namespace as renderers and other browser child processes.

Long term, it seems that the unique id generation should happen in //content/viz/host/ where the GpuHost implementation is moving to, in order to make it easier for all gpu clients use a single id namespace. Then we'd need a way to sync/map browser child process and GpuClient ids in content.

And, for the tracing id, short term, we can either pass in the tracing id along with the client id, or replicate its calculation in the GpuClient code. I'd prefer the former approach.

CL under review: https://crrev.com/c/1126488.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/694382d165b59085e7893d2571feb5116f2f2dc6

commit 694382d165b59085e7893d2571feb5116f2f2dc6
Author: Mohsen Izadi <mohsen@chromium.org>
Date: Tue Jul 10 19:21:32 2018

Remove GpuClientImpl dependecny on ChildProcessHostImpl

This is needed to remove GpuClient dependecny on content.
Implementation of GpuClient::Create() still depends on
ChildProcessHostImpl. That's not an issue as it would remain in content
even after GpuClientImpl is moved to //components/viz/host/, probably in
form of a standalone function.

BUG= 857216 
TEST=none

Change-Id: I725a1c13d60417c5b4da636d9789f7b989e9a42f
Reviewed-on: https://chromium-review.googlesource.com/1126488
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573853}
[modify] https://crrev.com/694382d165b59085e7893d2571feb5116f2f2dc6/content/browser/gpu/gpu_client_impl.cc
[modify] https://crrev.com/694382d165b59085e7893d2571feb5116f2f2dc6/content/browser/gpu/gpu_client_impl.h
[modify] https://crrev.com/694382d165b59085e7893d2571feb5116f2f2dc6/content/browser/renderer_host/render_process_host_impl.cc
[modify] https://crrev.com/694382d165b59085e7893d2571feb5116f2f2dc6/content/browser/service_manager/common_browser_interfaces.cc