Make gaia and oauth token fetchers recover from ERR_NETWORK_CHANGED.

Make gaia and oauth token fetchers recover from ERR_NETWORK_CHANGED.

This net error occurs frequently during ChromeOS sign-in when tokens are being
fetched, due to network change notifications triggered by the network library,
the portal detector and sometimes proxy configuration changes.

BUG=173671
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=180550

[Joao da Silva, 2013-02-01 17:40:08]

Zel,

I think this is the root cause of the recent bugs with sync not starting and
policy not fetching. It's similar to other bugs I've seen before and fixed in
December, and the recent rewrite of oauth2_policy_fetcher.cc started using a
GaiaAuthFetcher which doesn't retry on network changes.

The remaining fetchers in google_apis/gaia and chrome/browser/net/gaia are
already retrying on network changes.

Please review, thanks!

[Joao da Silva, 2013-02-04 19:14:57]

@Zel, can you have a look or assign to someone to review? Thanks!

[zel, 2013-02-04 19:31:14]

lgtm

[commit-bot: I haz the power, 2013-02-04 19:34:11]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/joaodasilva@chromium.org/12161003/1

[commit-bot: I haz the power, 2013-02-04 23:15:34]

Change committed as 180550