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As described on http://crbug.com/266955#c7

Also adding AtExitManager to gcapi_test.exe; this is required to support MasterPreferences's LazyInstance used by BrowserDistribution, used by google_update's ReadExperimentLabels().

Introducing GCAPITestRegistryOverrider as a class to be added as a member to GCAPI test fixtures that require registry overriding; extracted from the existing GCAPIReactivationTest fixture.

Move Windows-specific variations_util.cc code to experiment_labels_win.cc

BUG=266955
TEST=gcapi_test.exe

Review URL: https://codereview.chromium.org/23579003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233493 0039d316-1c4b-4281-b951-d872f2087c98