Add WebPrivateOwnPtr(const PassOwnPtr&) constructor like WebPrivatePtr.

WebPrivatePtr has WebPrivatePtr(const PassRefPtr&) constructor, so this CL adds
WebPrivateOwnPtr(const PassOwnPtr&) constructor.

WebTransformKeyframe uses this new constructor and removes the comment about
ownership transfer.

Review URL: https://codereview.chromium.org/26732007

git-svn-id: svn://svn.chromium.org/blink/trunk@159530 bbb929c8-8fbe-4397-9dbb-9b2b20218538