Update ChromeOS Build Instructions for GN

docs/chromeos_build_instructions.md

 # ChromeOS Build Instructions (Chromium OS on Linux)
 
 Chromium on Chromium OS is built on a mix of code sourced from Chromium
 on Linux and Chromium on Windows. Much of the user interface code is
 shared with Chromium on Windows. As such, if you make a change to
 Chromium on Windows you may find your changes affect Chromium on
 Chromium OS. Fortunately to test the effect of your changes you don't
 have to build all of Chromium OS, you can just build Chromium for
 Chromium OS directly on Linux.
 
 First, follow the [normal Linux build
 instructions](https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions.md)
 as usual to get a Chromium checkout.
 
-## Running Chromium on your local machine
+## Building and running Chromium with Chromium OS UI on your local machine
 
 If you plan to test the Chromium build on your dev machine and not a
-Chromium OS device run:
+Chromium OS device, run the following in your chromium checkout:
 
-```shell
-export GYP_DEFINES="chromeos=1"
-gclient runhooks
+    $ gn gen out/Default --args='target_os="chromeos"'
+    $ ninja -C out/Default
+
+NOTE: You may wish to replace 'Default' with something like 'Cros' if
+you switch back and forth between Linux and Chromium OS builds, or 'Debug'
+if you want to differentiate between Debug and Release builds (see below)
+or DebugCros or whatever you like.
+
+Now, when you build, you will build with Chromium OS features turned on.
+
+See [GN Build Configuration](https://www.chromium.org/developers/gn-build-configuration)
+for more information about configuring your build.
+
+Some additional options you may wish to set:
+```
+    is_component_build = true
+    use_goma = true
+    is_debug = false  # Release build
+    dcheck_always_on = true  # Enable DCHECK (with is_debug = false)
+    is_official_build = true
+    is_chrome_branded = true
 ```
 
-Now, once you build, you will build with Chromium OS features turned on.
-
-### Notes
+## Notes
 
 When you build Chromium OS Chromium, you'll be using the TOOLKIT\_VIEWS
 front-end just like Windows, so the files you'll probably want are in
 src/ui/views and src/chrome/browser/ui/views.
 
-If chromeos=1 is specified, then toolkit\_views=0 must not be specified.
+When target_os = "chromeos", then toolkit\_views need not (and should not)
+be specified.
 
 The Chromium OS build requires a functioning GL so if you plan on
 testing it through Chromium Remote Desktop you might face drawing
 problems (e.g. Aura window not painting anything). Possible remedies:
 
 *   --ui-enable-software-compositing --ui-disable-threaded-compositing
 *   --use-gl=osmesa, but it's ultra slow, and you'll have to build
     osmesa yourself.
 *   ... or just don't use Remote Desktop. :)
 
-Note the underscore in the GYP_DEFINES variable name, as people
-sometimes mistakenly write it GYPDEFINES.
-
 To more closely match the UI used on devices, you can install fonts used
 by Chrome OS, such as Roboto, on your Linux distro.
 
 To specify a logged in user:
 
 *   For first run, add the following options to the command line:
     **--user-data-dir=/tmp/chrome --login-manager**
 *   Go through the out-of-the-box UX and sign in as
     **username@gmail.com**
 *   For subsequent runs, add the following to the command line:
     **--user-data-dir=/tmp/chrome --login-user=username@gmail.com**.
 *   To run in guest mode instantly, you can run add the arguments
     **--user-data-dir=/tmp/chrome --bwsi --incognito
     --login-user='$guest' --login-profile=user**
 
 Signing in as a specific user is useful for debugging features like sync
 that require a logged in user.
 
-### Compile Testing Chromium with the Chromium OS SDK (quick version)
+## Compile Testing Chromium with the Chromium OS SDK (quick version)
 
-Note: These instructions are intended for Chromium developers trying to
-diagnose compile issues on Chromium OS, which can block changes in the
-CQ. See the [full
-documentation](http://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser)
-for more information about building & testing chromium for Chromium OS.
-
-To do a build of Chromium that can run on Chromium OS itself, the Chromium OS
-SDK must be used. The SDK provides all of chromium's dependencies as they are
-distributed with Chromium OS (as opposed to other distributions such as Ubuntu).
-
-To enter the SDK build environment, run the following command (replace the value
-of the `--board` flag with the name of the configuration you want to test).
-
-```shell
-cros chrome-sdk --board=amd64-generic --use-external-config
-```
-
-Once in the SDK build environment, build using the normal linux workflow (except
-for a different build directory):
-
-```shell
-gclient runhooks
-ninja -C out_amd64-generic/Release chromium_builder_tests
-```
-
-The current configurations verified by the CQ are:
-
-  Board Flag | Build Directory | CPU architecture
-  --- | --- | ---
-  amd64-generic | out_amd64-generic | 64-bit Intel
-  x86-generic | out_x86-generic | 32-bit Intel
-  daisy | out_daisy | 32-bit ARM
-
-## Running Chromium on a Chromium OS device
-
-Look at the [Chromium OS
-documentation](http://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser)
-for the official flow for doing this.
+See [Building Chromium for a Chromium OS device](https://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser)
+for information about building & testing chromium for Chromium OS.