Adds chrome.dial.fetchDeviceDecription API.

      This API fetches the device description for a DIAL device, given a label for a device already discovered.

The request is configured to not send/receive cookies, ignore proxies, and ignore redirects.

Requests are retried with backoff on 5XX responses.

The response is validated in multiple ways:
- Valid Application-URL: header value
- Response body is a valid UTF-8 string
- Response body is <= 256kb

Additional validations are possible by following the DIAL spec closely at the risk of ignoring some devices.  See comments in device_description_fetcher.cc for details.

Note: The API function implementation is complicated by thread hopping.  See comments in dial_api.h for how this may be improved in the future.

BUG=671932
TESTED=Manually, unit tests, API test

Review-Url: https://codereview.chromium.org/2583853004
Cr-Commit-Position: refs/heads/master@{#442713}