Simplify special case of EventTarget with hidden dependency action handling

This simplifies the most significant remaining special case for EventTarget
in the template code,
making both Python and Jinja much shorter.

Also a few Jinja formatting/whitespace fixes.

R=haraken
BUG=345503
TEST=fast/dom/event-target-arguments.html

Review URL: https://codereview.chromium.org/204133003

git-svn-id: svn://svn.chromium.org/blink/trunk@176904 bbb929c8-8fbe-4397-9dbb-9b2b20218538