Update docs for profiling Content Shell on Android.

docs/profiling_content_shell_on_android.md

 # Profiling Content Shell on Android
 
 Below are the instructions for setting up profiling for Content Shell on
 Android. This will let you generate profiles for ContentShell. This will require
 linux, building an userdebug Android build, and wiping the device.
 
 [TOC]
 
 ## Prepare your device.
 
@@ skipping 55 matching lines @@
 
 1.  Run `adb root`. Every time you connect your device you’ll want to run this.
 1.  If adb is not available, make sure to run `. build/android/envsetup.sh`
 
 If you get the error `error: device offline`, you may need to become a developer
 on your device before Linux will see it. On Jellybean 4.2.1 and above this
 requires going to “about phone” or “about tablet” and clicking the build number
 7 times:
 http://androidmuscle.com/how-to-enable-usb-debugging-developer-options-on-nexus-4-and-android-4-2-devices/
 
+## Enable profiling
+
+Rebuild `content_shell_apk` with profiling enabled.
+
+With GYP (deprecated):
+
+    export GYP_DEFINES="$GYP_DEFINES profiling=1"
+    build/gyp_chromium
+    ninja -C out/Release content_shell_apk
+
+With GN:
+
+    gn args out/Profiling
+    # add "enable_profiling = true"
+    ninja -C out/Profiling content_shell_apk
+    export CHROMIUM_OUTPUT_DIR="$PWD/out/Profiling"
+
 ## Run a Telemetry perf profiler
 
 You can run any Telemetry benchmark with `--profiler=perf`, and it will:
 
 1.  Download `perf` and `perfhost`
-1.  Install on your device
-1.  Run the test
-1.  Setup symlinks to work with the `--symfs` parameter
+2.  Install on your device
+3.  Run the test
+4.  Setup symlinks to work with the `--symfs` parameter
 
 You can also run "manual" tests with Telemetry, more information here:
 http://www.chromium.org/developers/telemetry/profiling#TOC-Manual-Profiling---Android
 
 The following steps describe building `perf`, which is no longer necessary if
 you use Telemetry.
 
+## Use `adb_profile_chrome`
+
+Even if you're not running a Telemetry test, you can use Catapult to
+automatically push binaries and pull the profile data for you.
+
+    build/android/adb_profile_chrome --browser=content_shell --perf
+
+While you still have to build, install and launch the APK yourself, Catapult
+will take care of creating the symfs etc. (i.e. you can skip the "not needed for
+Telemetry" steps below).
+
 ## Install `/system/bin/perf` on your device (not needed for Telemetry)
 
     # From inside the android source tree (not inside Chromium)
     mmm external/linux-tools-perf/
     adb remount # (allows you to write to the system image)
     adb sync
     adb shell perf top # check that perf can get samples (don’t expect symbols)
 
-## Enable profiling
-
-Rebuild `content_shell_apk` with profiling enabled
-
-    export GYP_DEFINES="$GYP_DEFINES profiling=1"
-    build/gyp_chromium
-    ninja -C out/Release content_shell_apk
-
 ## Install ContentShell
 
 Install with the following:
 
     build/android/adb_install_apk.py \
         --apk out/Release/apks/ContentShell.apk \
         --apk_package org.chromium.content_shell
 
 ## Run ContentShell
 
@@ skipping 43 matching lines @@
     chmod a+x perfhost_linux
 
 ## Actually record a profile on the device!
 
 Run the following:
 
     adb shell ps | grep content (look for the pid of the sandboxed_process)
     adb shell perf record -g -p 12345 sleep 5
     adb pull /data/perf.data
 
-
 ## Create the report
 
 1.  Run the following:
 
     ```
     ./perfhost_linux report -g -i perf.data --symfs symbols/
     ```
 
 1.  If you don’t see chromium/webkit symbols, make sure that you built/pushed
     Release, and that the symlink you created to the .so is valid!
@@ skipping 13 matching lines @@
 
     ```
     adb pull /proc/kallsyms symbols/kallsyms
     ```
 
 1.  Now add --kallsyms to your perfhost\_linux command:
     ```
     ./perfhost_linux report -g -i perf.data --symfs symbols/ \
         --kallsyms=symbols/kallsyms
     ```