Documentation: Update iOS build docs to include instructions for gn.

docs/ios_build_instructions.md

 # iOS Build Instructions
 
 **Note:** Upstreaming of iOS code is still a work in progress. In particular,
 note that **it is not currently possible to build an actual Chromium app.**
 Currently, the buildable binaries are ios\_web\_shell (a minimal wrapper around
 the web layer), and various unit tests.
 
 ## Prerequisites
 
 *   A Mac with a version of OS X capable of running the latest version
     of Xcode.
 *   The latest version of [Xcode](https://developer.apple.com/xcode/),
     including the current iOS SDK.
 *   The current version of the JDK (required for the closure compiler).
 *   [depot\_tools](http://dev.chromium.org/developers/how-tos/install-depot-tools).
 
 ## Setting Up
 
+### With GYP
+
 In the directory where you are going to check out the code, create a
 `chromium.gyp_env` to set the build to use iOS targets (and to use
 hybrid builds; see [Building](#Building) below):
 
 ```shell
 cat > chromium.gyp_env <<EOF
 {
   "GYP_DEFINES": "OS=ios",
   "GYP_GENERATORS": "ninja,xcode-ninja",
 }
 EOF
 ```
 
 If you aren't set up to sign iOS build products via a developer account,
 you should instead use:
 
 ```shell
 cat > chromium.gyp_env <<EOF
 {
   "GYP_DEFINES": "OS=ios chromium_ios_signing=0",
   "GYP_GENERATORS": "ninja,xcode-ninja",
 }
 EOF
 ```
 
+### With GN
+
+Use `gn args out/Debug-iphonesimulator` (or replace
+`out/Debug-iphonesimulator` with your chosen `out/` directory) to open up an
+editor to set the following gn variables and regenerate:
+
+```
+# Set to true if you have a valid code signing key.
+ios_enable_code_signing = false
+target_os = "ios"
+# Set to "x86", "x64", "arm", "armv7", "arm64". "x86" and "x64" will create a
+# build to run on the iOS simulator (and set use_ios_simulator = true), all
+# others are for an iOS device.
+target_cpu = "x64"
+# Release vs debug build.
+is_debug = true
+```
+
+### API Keys
+
 Before you build, you may want to
 [install API keys](https://sites.google.com/a/chromium.org/dev/developers/how-tos/api-keys)
 so that Chrome-integrated Google services work. This step is optional if you
 aren't testing those features.
 
 ## Getting the Code
 
 Next, [check out the
 code](https://www.chromium.org/developers/how-tos/get-the-code), with:
 
@@ skipping 39 matching lines @@
 ```
 
 ## Converting an existing Mac checkout into an iOS checkout
 
 If you want to convert your Mac checkout into an iOS checkout, follow the steps
 below:
 
 1.  Add `target_os = [ "ios" ]` to the bottom of your `chromium/.gclient`
 file.
 
-2.  Make sure you have the following in your `chromium/chromium.gyp_env`
-file (removing the `chromium_ios_signing=0` if you want to make
-developer-signed builds):
+2.  For gyp, make sure you have the following in your
+`chromium/chromium.gyp_env` file (removing the `chromium_ios_signing=0` if you
+want to make developer-signed builds):
 
     ```json
     {
       "GYP_DEFINES" : "OS=ios chromium_ios_signing=0",
       "GYP_GENERATORS" : "ninja,xcode-ninja",
     }
     ```
 
+    For gn, add the arguments specified [above](#With-GN) to your gn setup.
+
 3.  Make sure to sync again to fetch the iOS specific dependencies and
 regenerate build rules using:
 
     ```shell
     gclient sync
     ```
 
 ## Troubleshooting
 
 If your build fails, check the iOS columns of [the Mac
 waterfall](http://build.chromium.org/p/chromium.mac/console) (the last two) to
 see if the bots are green. In general they should be, since failures on those
 bots will close the tree.