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This creates a new thread for polling evdev on, and moves the
InputDeviceFactoryEvdev object onto that thread. We'll use this new
thread to avoid UI-thread jank affecting the cursor.

BUG=449710
TEST=boot link_freon & move cursor with simulated UI jank.
     cursor moves while UI thread is stalled.

Review URL: https://codereview.chromium.org/873833002

Cr-Commit-Position: refs/heads/master@{#313981}