clientcerts: simplify SSLKeyConverter

Previously SSLKeyConverter was a class with two implementations:
SSLKeyConverterImpl and StubSSLKeyConverter. It was used as a test shim
so that unit tests could avoid dealing with actual hardware-backed keys
(which would not work or be very slow). It had two methods:

  ConvertUnexportableKeySlowly(UnexportableSigningKey&)
  ConvertECKey(ECPrivateKey&)

The second method doesn't make sense though - ECPrivateKey is never
hardware-backed, and converting it doesn't need to be mocked for tests
because the "conversion" process is a no-op that just wraps the key into
another object. In fact, both implementations of ConvertECKey() had
identical code.

This change therefore removes SSLKeyConverter, replacing it with a
single free function:

  SSLPrivateKeyFromUnexportableSigningKeySlowly(UnexportableSigningKey&)

and the test faking behavior is replaced with a single callback which
tests can set to a conversion function to replace the default behavior.
This simplifies the code a bit and avoids the need for allocating a
fresh SSLKeyConverter object every time a key was to be converted.

This refactor is in preparation for migrating the client cert code from
crypto/ec_private_key (which is deprecated) to crypto/keypair (which is
not).

Bug: 372283556
Change-Id: I16a62c214c53fc0a301f4d40b14520bcd05497f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6542255
Reviewed-by: Sébastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460783}