Initialize OSCrypt with a TaskRunner on the main thread

Gnome-Keyring requires calls to originate from the main thread. In order
to support Gnome-Keyring on OSCrypt, we initialize it with a SingleThreadTaskRunner.

In addition, the various static fields for OSCrypt's initialization
have been grouped into a struct.

BUG=602624

Review-Url: https://codereview.chromium.org/2262693002
Cr-Commit-Position: refs/heads/master@{#413110}
5 files changed