SkiaLab doc: add another step for new Android devices

site/dev/testing/skialab.md

 SkiaLab
 =======
 
 Overview
 --------
 
 Skia's buildbots are hosted in three places:
 
 * Google Compute Engine. This is the preferred location for bots which don't
   need to run on physical hardware, ie. anything that doesn't require a GPU,
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 1. Locate or add a host machine. We generally want to keep the number of
    devices attached to each host below 5 or so. If a new host machine is
    required, follow the above instructions for bringing up a new buildbot
    host machine, with the exception that the slave corresponds to the Android
    device, not the host machine itself.
 2. Ensure that the buildslave is not yet running:
 
         $ killall python
 
-3. Connect the device to the host machine, either through a powered USB hub or
+3. Disable MTP and PTP on the device.  Some devices require one or the other to
+   be enabled; in that case, select PTP and choose to 'do nothing' when
+   attaching to the host machine.
+4. Connect the device to the host machine, either through a powered USB hub or
    directly to the machine.
-4. Make sure that the device is in developer mode and that USB debugging is
+5. Make sure that the device is in developer mode and that USB debugging is
    enabled.
-5. Authorize the device for USB debugging on the host machine by checking the
+6. Authorize the device for USB debugging on the host machine by checking the
    "always allow" box on dialog box which appears on the Android device after
    plugging it into the host.
-6. Ensure that the device appears as "connected" when you run the
+7. Ensure that the device appears as "connected" when you run the
    `which_devices.py` script:
 
         $ python buildbot/scripts/which_devices.py
 
-7. Reboot the machine to start the buildslave.
+8. Reboot the machine to start the buildslave.
 
 
 ### Bringing up a new machine from scratch
 
 TODO(borenet): Migrate from Google Docs.
 
 OS-specific instructions are available in a
 [Google Doc](https://docs.google.com/document/d/1X7Hvsj33AlBmj-KEWfFbmdCArUJJAICLkB7ipDcxRV8/edit)
 
 
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 11. Select the external drive to use for /home/partimag, something like,
     "1000GB_ntfs_My_Passport".
 12. Select the bot_img directory.
 13. "Beginner"
 14. "savedisk"
 15. Choose a name for the disk image. The convention is:
     `skiabot-<hardware type>-<OS>-<disk image revision #>`
 12. Choose the hard drive in the machine. It should be the only option.
 13. "y"
 14. Choose "reboot" or "shut down" when finished.