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Serializing calls to getAuthToken will eliminate unnecessary calls to
GAIA and consolidate UI flows.

There is are two queues of requests for each (extension, scopes)
tuple, one for the non-interactive IssueToken flow and one for the
interactive prompt.

BUG=228908
(step #1 of the bug description)

Review URL: https://codereview.chromium.org/14270007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194627 0039d316-1c4b-4281-b951-d872f2087c98