Intorude DEFINE_NODE_TYPE_CASTS macro to make type cast functions easily.

It's similar to DEFINE_CSS_VALUE_TYPE_CASTS in Source/core/css/CSSValue.h.
Apply DEFINE_NODE_TYPE_CASTS to all of Node subclasses in Source/core/dom/.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/26383003

git-svn-id: svn://svn.chromium.org/blink/trunk@159446 bbb929c8-8fbe-4397-9dbb-9b2b20218538