Cleanup FakeWebPlugin.[h|cpp].

1. Remove unnecessary forward declaration and #include.
2. Fix a wrong namespace comment for consistency.

Review URL: https://codereview.chromium.org/595793002

git-svn-id: svn://svn.chromium.org/blink/trunk@182524 bbb929c8-8fbe-4397-9dbb-9b2b20218538