Make sure getAttribute() / setAttribute() callers use AtomicStrings

Make sure getAttribute() / setAttribute() callers use AtomicStrings. Especially,
the DOM API should make sure to request an AtomicString if they call setAttribute()
internally so that the bindings code won't uselessly created a WTF::String from the
v8::String. That String will then implicitly be converted to an AtomicString when
calling setAttribute(), which has a cost.

This is one step towards making the AtomicString(const String&) constructor
explicit.

R=abarth, eseidel, haraken
BUG=323739

Review URL: https://codereview.chromium.org/99333011

git-svn-id: svn://svn.chromium.org/blink/trunk@163293 bbb929c8-8fbe-4397-9dbb-9b2b20218538