Change the key type of CheckedRadioButtons::NameToGroupMap: StringImpl* -> AtomicString.

We had to manage the map lifetime and HTMLInputElement, which owns the
StringImpl, lifetime strictly. It made some optimization harder. Using
AtomicString is much simpler.

BUG=334556
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/129873022

git-svn-id: svn://svn.chromium.org/blink/trunk@165622 bbb929c8-8fbe-4397-9dbb-9b2b20218538