Fix a race in the creation of HidConnectionLinux::Helper.

Fix a race in the creation of HidConnectionLinux::Helper.

The creation of this object on the FILE thread races with the possible
destruction of the object on the UI thread. The scoped_ptr<> that owns
the pointer may try to destroy the object on the UI thread if the
connection is closed before the helper is started. Generally, updating
the helper_ pointer from the FILE thread is a bad idea.

This patch moves creation of the helper to the UI thread, it is then
detached from that thread when it is started. The HidConnectionLinux no
longer uses a scoped_ptr<> to hold it and instead must always free it
explicitly with DeleteSoon.

BUG=

Review URL: https://codereview.chromium.org/786343003

Cr-Commit-Position: refs/heads/master@{#307850}