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This adds a --pen-devices flag that works the same way as
--touch-devices and makes it possible to simulate pen
devices on Linux in addition to the current support for
touch devices.

Bug: 
Test: manual
Change-Id: Icc103b029b91b1e9e5d12e89c594069da38577ed
Reviewed-on: https://chromium-review.googlesource.com/667719
Commit-Queue: David Reveman <reveman@chromium.org>
Reviewed-by: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502918}