Use WebPrivateOwnPtr instead of raw pointer in WebBlobData and WebFilterKeyframe.

WebPrivateOwnPtr requires calling reset() in Blink before WebPrivateOwnPtr is
destructed. Both WebFilterKeyframe and WebBlobData are always destructed in
Blink, so both destructors can call WebPrivateOwnPtr::reset().

initialize() and reset() of WebPrivateOwnPtr are removed because
1. Both are not used.
2. Both makes it hard to use WebPrivateOwnPtr.

Review URL: https://codereview.chromium.org/26729003

git-svn-id: svn://svn.chromium.org/blink/trunk@159775 bbb929c8-8fbe-4397-9dbb-9b2b20218538