Remove unnecessary static_cast<SocketStreamHandle*>().

|this| is SocketStreamHandle*. These static_casts are unnecessary.

BUG=

Review URL: https://codereview.chromium.org/105513004

git-svn-id: svn://svn.chromium.org/blink/trunk@163501 bbb929c8-8fbe-4397-9dbb-9b2b20218538