Use scoped_refptr<SingleThreadTaskRunner> when initializing ProxyConfigService

Previously MessageLoop was used to pass reference to the FILE thread
to ProxyConfigServiceLinux. Changeed it to use SingleThreadTaskRunner.
Also for other thread the task runners were passed as raw pointers.
Changed it to use scoped_refptr<> instead.

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Review URL: https://codereview.chromium.org/540593002

Cr-Commit-Position: refs/heads/master@{#293395}
14 files changed