Enable arm/linux chromium to build with gn

docs/linux_chromium_arm.md

 # Linux Chromium Arm Recipes
 
 [TOC]
 
 ## Recipe1: Building for an ARM CrOS device
 
-This recipe uses `ninja` (instead of `make`) so its startup time is much lower
-(sub-1s, instead of tens of seconds), is integrated with goma (for
-google-internal users) for very high parallelism, and uses `sshfs` instead of
-`scp` to significantly speed up the compile-run cycle. It has moved to
 https://sites.google.com/a/chromium.org/dev/developers/how-tos/-quickly-building-for-cros-arm-x64
-(mostly b/c of the ease of attaching files to sites).
-
 
 ## Recipe2: Explicit Cross compiling
 
 Due to the lack of ARM hardware with the grunt to build Chromium native, cross
 compiling is currently the recommended method of building for ARM.
 
 These instruction are designed to run on Ubuntu Precise.
 
 ### Installing the toolchain
 
 The install-build-deps script can be used to install all the compiler
 and library dependencies directly from Ubuntu:
 
     $ ./build/install-build-deps.sh
 
 ### Installing the sysroot
 
 A prebuilt sysroot image is kept up-to-date on Cloud Storage. It will
 automatically be installed by gclient runhooks installed if you have
 `target_arch=arm` in your `GYP_DEFINES`.
 
 To install the sysroot manually you can run:
 
     ./chrome/installer/linux/sysroot_scripts/install-debian.wheezy.sysroot.py \
         --arch=arm
 
 ### Building
 
 To build for ARM, using the clang binary in the chrome tree, use the following
-settings:
+gn args:
 
-    export GYP_CROSSCOMPILE=1
-    export GYP_DEFINES="target_arch=arm"
+    target_cpu = "arm"
 
-There variables need to be set at gyp-time (when you run `gyp_chromium`),
-but are not needed at build-time (when you run make/ninja).
+Or the following gyp settings:
+
+    GYP_CROSSCOMPILE=1
+    GYP_DEFINES="target_arch=arm"
 
 ## Testing
 
 ### Automated Build and Testing
 
 Chromium's testing infrastructure for ARM/Linux is somewhat limited.
 There are currently two builders setup, one on the FYI waterfall and one
 the the trybot waterfall:
 
 *   [Linux ARM](http://build.chromium.org/p/chromium.fyi/builders/Linux%20ARM)
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 ### Notes
 
 *   To building for thumb reduces the stripped release binary by around 9MB,
     equating to ~33% of the binary size.  To enable thumb, set `'arm_thumb': 1`
 *   TCmalloc does not have an ARM port, so it is disabled.