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/566373002

git-svn-id: svn://svn.chromium.org/blink/trunk@182133 bbb929c8-8fbe-4397-9dbb-9b2b20218538