Use type cast macro for AnimatableLengthPoint|3D instead of manual toFoo()

Type casting generation macro supports more plenty toFoo() function with
unnecessary type cast checking.

Additionally this cl clean up static_cast<CSS|HTML|AXFoo>
using toFoo().

BUG=309516

Review URL: https://codereview.chromium.org/370073002

git-svn-id: svn://svn.chromium.org/blink/trunk@177624 bbb929c8-8fbe-4397-9dbb-9b2b20218538