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This CL introduces the PrefHashBrowserTestBase fixture which these tests are based on.

This fixture (in conjunction with the new PREF_HASH_BROWSER_TEST macro) provides an easy way to override 3 methods in order to:
 1) Setup Chrome as desired in a PRE_ test.
 2) Attack Preferences while Chrome isn't running.
 3) Relaunch Chrome and verify reaction to attacks.

The fixture+macro also parametrizes every test such that they are ran in every SettingsEnforcement trial group.

A few pieces were resurrected from the old PrefHashBrowserTest used to test unloaded profile seeding prior to http://crrev.com/277209

Review URL: https://codereview.chromium.org/431973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287689 0039d316-1c4b-4281-b951-d872f2087c98