exo: Fix crash in out-of-order destruction of Gamepad

exo: Fix crash in out-of-order destruction of Gamepad

Destroying the gaming input resource will destroy the gamepad
polling thread, which causes the Gamepad destructor to access an
invalid reference to the task runner of that thread.
This CL changes the raw pointer to a ref counted pointer so we won't
destroy the task runner prematurely.

BUG=620977
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2159283002
Cr-Commit-Position: refs/heads/master@{#406327}
(cherry picked from commit a3633143505b30713e83baabfa7767877bf79788)

[reveman, 2016-07-25 21:31:13]

reveman@chromium.org changed reviewers:
+ reveman@chromium.org

[reveman, 2016-07-25 21:31:13]

lgtm

[denniskempin, 2016-07-25 21:31:47]

The CQ bit was checked by denniskempin@google.com

[commit-bot: I haz the power, 2016-07-25 21:33:01]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-25 21:34:25]

Committed patchset #1 (id:1)