network service: Fix README about threading model

- Add a link to the code comment.
- Remove inaccuracy: in-process doesn't always run on the IO thread

Change-Id: I827b8646ce448a86d3383a954e8835c20676d547
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2223708Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#778612}

services/network/README.md

@@ -48,9 +48,11 @@ can also be useful for debugging; for example, it's used in Chromium's
 mode.
 
 *In the out-of-process case*: The network service runs on the [IO
-thread](/docs/threading_and_tasks.md) of the utility process. The utility
-process houses only the network service, so there is nothing running on its main
-thread.
+thread](/docs/threading_and_tasks.md) of the utility process (see this
+[comment][1] in `content/utility/services.cc` for why). The utility process
+houses only the network service, so there is nothing running on its main thread.
+
+[1]: https://source.chromium.org/chromium/chromium/src/+/master:content/utility/services.cc;l=197-198;drc=9b85cd82c52e13ed685dd74c726d91067bbd34d5
 
 *In the in-process case*: The network service runs on its own dedicated thread
 in the browser process. Exception: on Chrome OS, it currently runs on the IO
@@ -66,8 +68,8 @@ content/utility/services.cc. This calls `RunNetworkService()` which creates the
 `network::NetworkService` instance. For more background about Chromium's
 services architecture, see [Mojo and Services](/docs/mojo_and_services.md).
 
-*In the in-process case*: The browser starts the network service on the IO
-thread. See `CreateInProcessNetworkService` in
+*In the in-process case*: The browser process starts the network service. See
+`CreateInProcessNetworkService()` in
 `content/browser/network_service_instance_impl.cc`, which posts a task to create
 the `network::NetworkService` instance.