Have Document::parseQualifiedName() deal with AtomicStrings

Have Document::parseQualifiedName() deal with AtomicStrings instead of Strings as
its purpose is to extract a prefix and localName that the caller can use to
construct a QualifiedName object. The QualifiedName constructor takes
AtomicStrings in argument so it does not make sense for parseQualifiedName() to
construct and return Strings. The caller will always need to convert these to
AtomicStrings, which has a cost.

R=abarth, eseidel
BUG=323739

Review URL: https://codereview.chromium.org/108483002

git-svn-id: svn://svn.chromium.org/blink/trunk@163355 bbb929c8-8fbe-4397-9dbb-9b2b20218538