Remove EventTarget subclass boilerplate using a REFCOUNTED_EVENT_TARGET macro

Anyone writing a subclass of EventTarget previously had to go through a C++
dance of adding "using" declarations in their class along with boilerplate
overloads of EventTarget::{ref,deref}EventTarget methods. The macro added
in this patch replaces that dance, and comments have been added to aid
Blink developers in using these new features.

R=ch.dumez@samsung.com

Review URL: https://codereview.chromium.org/27073004

git-svn-id: svn://svn.chromium.org/blink/trunk@159796 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move KeyCodeConversion test near its implementation file.

BUG=None
TEST=blink_platform_unittests, no behavior changes
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27137002

git-svn-id: svn://svn.chromium.org/blink/trunk@159797 bbb929c8-8fbe-4397-9dbb-9b2b20218538