Remove default key slot from KeygenHandler.
KeygenHandler defaulted to use the persistent key slot. However, the Handler is also used in ChromeOS where no such global persistent slot exists.
This change removes this default behavior from KeygenHandler.
Instead only the slot provided by the CryptoModuleDelegate will be used.
If no CryptoModuleDelegate is set, then an error will be thrown on keygen usage.
The unit test of KeygenHandler now explicitly provides the slot through a StubCryptoModuleDelegate.
In a follow up, the crypto::GetPersistentNSSKeySlot() call will be removed from the unit tests as well and instead the test slot will be get()'ed from the ScopedTestNSSDB.
BUG=210525
Review URL: https://codereview.chromium.org/
384413004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283634 0039d316-1c4b-4281-b951-d872f2087c98