Bindings: use |holder| local variable in attribute getters and setters

Current generated bindings for attribute getters has a
special case for |holder|, as this is sometimes stored as a
local variable.
This simplifies the template flow.

R=haraken
BUG=345503

Review URL: https://codereview.chromium.org/214143003

git-svn-id: svn://svn.chromium.org/blink/trunk@170143 bbb929c8-8fbe-4397-9dbb-9b2b20218538