Clean up static_cast<const toBasicShapeFoo*> by using toBasicShapeFoo()

Removed remained static_cast<const toBasicShapeFoo*>.

BUG=309516

Review URL: https://codereview.chromium.org/381473006

git-svn-id: svn://svn.chromium.org/blink/trunk@177811 bbb929c8-8fbe-4397-9dbb-9b2b20218538