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This change updates the SecurityKeyAuthHandlerLinux class to run its
socket operations on a task_runner which allows blocking IO.

The first part of the change is plumbing through the file_task_runner
from the Me2Me host (which owns the context with the task runners)
down to the auth handler class which requires it.

The second part of the change was to move the file delete operation
to the file thread and then post back to the main thread.

I've also updated the unit tests for the affected class since it now
needs to handle synchronization between two threads.

BUG=591739
R=sergeyu@chromium.org
TBR=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2168303003
Cr-Commit-Position: refs/heads/master@{#407649}