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until after session restore has completed.

This is mainly to help login/startup times so that Chrome is useful to
the user earlier.  To make sure that this is actually helping, this CL
includes a Finch experiment that will only enable the deferral on 50%
of the clients.

It is also expected that deferring these will help with some problems
we've seen when extensions attempt to do GAIA authentication.

BUG=279427,259791
TEST=Ran performance tests on ChromeOS and Linux

Review URL: https://chromiumcodereview.appspot.com/23618014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221621 0039d316-1c4b-4281-b951-d872f2087c98