Use xmlAtom / starAtom when possible instead of string literals

Use xmlAtom / starAtom when possible instead of string literals. When comparing
to another AtomicString, this avoids doing a character by character comparison,
as we can do a faster pointer comparison instead. This also gets rid of a
redundant static variable.

R=eseidel

Review URL: https://codereview.chromium.org/108683002

git-svn-id: svn://svn.chromium.org/blink/trunk@163359 bbb929c8-8fbe-4397-9dbb-9b2b20218538