Add ascii() / latin1() / utf8() methods to AtomicString to avoid having to call string()

Add ascii() / latin1() / utf8() methods to AtomicString to avoid having to call
string() first. These methods just proxy the calls to the internal String
member.

R=eseidel

Review URL: https://codereview.chromium.org/124003003

git-svn-id: svn://svn.chromium.org/blink/trunk@164482 bbb929c8-8fbe-4397-9dbb-9b2b20218538