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

docs/ios_build_instructions.md

Index: docs/ios_build_instructions.md
diff --git a/docs/ios_build_instructions.md b/docs/ios_build_instructions.md
index d609e944ec22d2eac61fa90f423bde0331662c7f..ddaade6399af658ef916a59093cf00bd5d53160b 100644
--- a/docs/ios_build_instructions.md
+++ b/docs/ios_build_instructions.md
@@ -16,6 +16,8 @@ the web layer), and various unit tests.
 
 ## 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):
@@ -41,6 +43,22 @@ cat > chromium.gyp_env <<EOF
 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:
+
+```
+# For the iOS simulator at out/Debug-iphonesimulator/iossim.
+use_ios_simulator = true

[sdefresne, 2016/04/22 15:22:37]

This is not required as the value of use_ios_simulator is automatically
initialized from target_cpu on iOS.
I usually tweak the following settings on iOS:

ios_enable_code_signing = false
is_debug = true
target_os = "ios"
target_cpu = "x64"

Then is_debug control release/debug while target_cpu can take the values "x86",
"x64", "arm", "armv7" or "arm64". The first two implies simulator build, the
last one implies device build. ios_enable_code_signing should be set to false if
you do not have valid code signing key.

[Patti Lor, 2016/04/26 00:03:01]

Done, thanks for the extra information!

+# Set to true for developer-signed builds.
+ios_enable_code_signing = false
+target_os = "ios"
+```
+
+### 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
@@ -100,9 +118,9 @@ 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
     {
@@ -111,6 +129,8 @@ developer-signed builds):
     }
     ```
 
+    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: