WebMIDIClientMock cleanups

- Remove unnecessary forward declaration. That is part of WebMIDIClient
interface.
- Remove the static create function.
- Just expose the constructor instead.

BUG=None
TEST=webkit_unit_tests
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/105403005

git-svn-id: svn://svn.chromium.org/blink/trunk@163405 bbb929c8-8fbe-4397-9dbb-9b2b20218538