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|ChromeOSOAuth2TokenServiceDelegate| should pre-emptively reject access
token fetches and other operations on known dummy Gaia tokens instead of
letting them go over the network and get rejected by Gaia server-side.

This brings feature parity between |ChromeOSOAuth2TokenServiceDelegate|
and |MutableProfileOAuth2TokenServiceDelegate|, which does the same
thing.

Bug: 915628
Test: unit_tests --gtest_filter="*CrOSOAuthDelegateTest*"
Change-Id: I812a847fb1afd040a3ae743a9e491753878ca523
Reviewed-on: https://chromium-review.googlesource.com/c/1437627
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627034}