Remove direct Host creation from RebaselineServer.

Remove Host() call from RebaselineServer. The host object is passed
to the RebaselineServer from outside.
This could help with the Mocking as the FIXME mentioned.

BUG=None
TEST=test-webkitpy

Review URL: https://codereview.chromium.org/561733003

git-svn-id: svn://svn.chromium.org/blink/trunk@181893 bbb929c8-8fbe-4397-9dbb-9b2b20218538