Use EventTargetWithInlineData instead of EventTarget

Inherit from EventTargetWithInlinedata rather than EventTarget,
which was introduced at r159393, to reduce redundant overrides.

BUG=none
TEST=pass all tests
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/27634002

git-svn-id: svn://svn.chromium.org/blink/trunk@159830 bbb929c8-8fbe-4397-9dbb-9b2b20218538