Use ConstructFromLiteral instead of implicit conversion

Since we are supposed to end up with an AtomicString, don't rely on implicit conversion, but provide it as static local variable while using ConstructFromLiteral to avoid using strlen.

Review URL: https://codereview.chromium.org/567653002

git-svn-id: svn://svn.chromium.org/blink/trunk@181840 bbb929c8-8fbe-4397-9dbb-9b2b20218538