Make sure we correctly dispatch custom events having the same names as unprefixed animations events

If support for unprefixed animations is disabled and a custom event happens
to have the same type name then we should dispatch them no matter.

As inside blink we dispatch unprefixed events (no matter if the support is on
or off), we had a piece of code which was retargeting the name to unprefixed when
the support was off. This was not working in the case of custom events as they were
being retargeted to prefixed. I'm glad we can get rid of this condition when the
support of unprefixed animations ships.

I've added a set of tests in the stable/ directory to specifically covers the fact
that the unprefixed animations support is off. These two tests can be removed when
unprefixed animations are default. The other tests in the regular animations/ directory
are for regression testing.

BUG=367537

Review URL: https://codereview.chromium.org/263553010

git-svn-id: svn://svn.chromium.org/blink/trunk@173024 bbb929c8-8fbe-4397-9dbb-9b2b20218538