network service: Expand README.

Add more information about the process/threading model of network
service and how it starts.

Change-Id: I0669d6d4cb96eddff494f7c08ac7db575a628cee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215523
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772137}

services/network/README.md

@@ -12,7 +12,7 @@ Some design goals
     not have hooks here. The only exception is when it's impossible for these
     features to function without some hooks in the network service. In that
     case, we add the minimal code required. Some examples included traffic
-    shaping for devtools and CORB blocking.
+    shaping for devtools, CORB blocking, and CORS.
   * every PostTask, thread hop and process hop (IPC) should be counted carefully
     as they introduce delays which could harm this performance critical code.
   * `NetworkContext` and `NetworkService` are trusted interfaces that aren't
@@ -28,6 +28,60 @@ https://docs.google.com/document/d/1wAHLw9h7gGuqJNCgG1mP1BmLtCGfZ2pys-PdZQ1vg7M/
 
 [URLLoader](url_loader.md)
 
+# Where does the network service run?
+
+> Note: For more background about this section, see also [Multi-process
+Architecture](https://www.chromium.org/developers/design-documents/multi-process-architecture)
+for an overview of the processes in Chromium.
+
+The network service is designed as a [Mojo service](/docs/mojo_and_services.md)
+that in general doesn't need to be aware of which thread/process it runs on.
+The browser process launches the network service and decides whether to run it
+inside the browser process (*in-process*) or in a dedicated utility process
+(*out-of-process*).
+
+The out-of-process configuration is preferred for isolation and stability, and
+is the default on most platforms. The in-process configuration is the default on
+Android because of some unresolved issues; see https://crbug.com/1049008.  It
+can also be useful for debugging; for example, it's used in Chromium's
+[`--single-process`](https://www.chromium.org/developers/design-documents/process-models)
+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.
+
+*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
+thread; see https://crbug.com/1086738.
+
+# How does the network service start?
+
+*In the out-of-process case*: The browser creates the utility process and asks
+it to launch the network service. For the browser-side code, see
+`GetNetworkService()` in `content/browser/network_service_instance_impl.cc`.
+For the utility process code, see `GetIOThreadServiceFactory` in
+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
+`content/browser/network_service_instance_impl.cc`, which posts a task to create
+the `network::NetworkService` instance.
+
+# What happens if the network service crashes?
+
+*In the out-of-process case*: If the network service crashes, it gets restarted
+in a new utility process. The goal is for the failure to be mostly recoverable:
+some in-flight requests may fail, but any tabs open will continue to function
+and later requests will succeed.
+
+*In the in-process case*: If the network service crashes in this case, of
+course, the entire browser crashes. This is one reason for the goal to always
+run it out-of-process.
+
 # Buildbot
 
 The [Network Service