cryptohome: Merge FakeCryptohomeClient and CryptohomeClinentStubImpl

The new FakeCryptohomeClient is based on CryptohomeClinentStubImpl.
The only addition is set_unmout_result().

BUG=298729
TEST=the existing tests should pass
R=hashimoto@chromium.org, stevenjb@chromium.org

Review URL: https://codereview.chromium.org/24637004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225664 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225664