Consistently use AtomicString for namespaceURI / prefix

Consistently use AtomicString for namespaceURI / prefix. In particular, Node's
lookupPrefix(), lookupNamespaceURI(), lookupNamespacePrefix() now deal with
AtomicStrings.

This avoids needless creation of Strings when we actually need AtomicStrings.
This is one step towards marking the AtomicString(const String&) constructor
explicit.

R=eseidel, haraken
BUG=323739

Review URL: https://codereview.chromium.org/108543003

git-svn-id: svn://svn.chromium.org/blink/trunk@163446 bbb929c8-8fbe-4397-9dbb-9b2b20218538