Use explicit HTMLFrameElementBase cast

As <https://chromiumcodereview.appspot.com/19504005/diff/1/Source/core/rendering/RenderLayer.cpp#oldcode2247> mentioned,
it should be HTMLFrameElementBase that embraces both HTMLFrameElement and HTMLIFrameElement.

Review URL: https://chromiumcodereview.appspot.com/19654003

git-svn-id: svn://svn.chromium.org/blink/trunk@154469 bbb929c8-8fbe-4397-9dbb-9b2b20218538