Update chromeos_build_instructions for GN and clean up.

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
+Chromium on Chromium OS is built from 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
+shared with Chromium on Windows.
+
+If you make changes to Chromium on Windows, they may affect Chromium
+on Chromium OS. Fortunately to test the effects of your changes you
+don't need 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.
 
 ## 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 the following in your chromium checkout:
 
     $ 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:
-```
+If you have not already done so, be sure to set the following to prevent
+'gclient runhooks' from executing 'gyp_chromium':
+
+    export GYP_CHROMIUM_NO_ACTION=1
+
+Some additional options you may wish to set by passing in **--args** to
+**gn gen** or running **gn args out/Default**:
+
     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
-```
 
 ## 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.
 
 When target_os = "chromeos", then toolkit\_views need not (and should not)
 be specified.
 
@@ skipping 17 matching lines @@
     **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 Chromium for a Chromium OS device using the Chromium OS SDK
 
 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.
+for information about building and testing chromium for Chromium OS.