revoke unused OAuth2 tokens on signout and re-signin

revoke unused OAuth2 tokens on signout and re-signin 

A number of users have been reporting problems with the identity API. Gaia investigations reveal that these users have overflowed a limit on the number of login tokens they can have for Chrome. Revoking tokens we don't need any longer should mitigate the problem. 

This is identical to https://codereview.chromium.org/13249007/ but with a corrected base url.

BUG=224462
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=192491

[Michael Courage, 2013-04-04 00:11:40]

Tim or Roger, can one of you rubber stamp this change? It's identical to
13249007, but I had to create a new change to fix the base URL so the CQ will
take it.

[Roger Tawa OOO till Jul 10th, 2013-04-04 12:50:34]

lgtm

[commit-bot: I haz the power, 2013-04-04 17:20:16]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/courage@chromium.org/13599003/1

[commit-bot: I haz the power, 2013-04-04 17:22:48]

Step "update" is always a major failure.
Look at the try server FAQ for more details.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=mac_rel&nu...

[commit-bot: I haz the power, 2013-04-04 18:43:20]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/courage@chromium.org/13599003/1

[commit-bot: I haz the power, 2013-04-04 18:45:57]

Step "update" is always a major failure.
Look at the try server FAQ for more details.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=ios_dbg_si...

[commit-bot: I haz the power, 2013-04-04 23:02:03]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/courage@chromium.org/13599003/1

[commit-bot: I haz the power, 2013-04-05 04:03:03]

Change committed as 192491