Support SVG in event.path.

The SVG instance tree was recently removed which also removed most of the gnarly event handling in SVG. Now we can support SVG also in event.path.

BUG=402721

Review URL: https://codereview.chromium.org/477883003

git-svn-id: svn://svn.chromium.org/blink/trunk@180364 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use an AtomicString as key in DocumentOrderedMap

Use an AtomicString as key in DocumentOrderedMap instead of a StringImpl*.
This is safer as the HashMap Traits rely on the StringImpl* to not be a
dangling pointer (it is calling StringImpl::hash()). Using an AtomicString
as key will make sure we don't experience use-after-free such as in Bug
402255.

I ran Bindings/get-element-by-id.html test and did not see any performance
regression.

R=esprehn@chromium.org, morrita@chromium.org
BUG=402255

Review URL: https://codereview.chromium.org/467973003

git-svn-id: svn://svn.chromium.org/blink/trunk@180365 bbb929c8-8fbe-4397-9dbb-9b2b20218538