Use v8AtomicString instead of v8::String::NewFromUtf8

We should use utility methods as much as possible.

Review URL: https://codereview.chromium.org/116983005

git-svn-id: svn://svn.chromium.org/blink/trunk@164510 bbb929c8-8fbe-4397-9dbb-9b2b20218538