V8StringResource's v8StringToWebCoreString() should return a StringType

V8StringResource's v8StringToWebCoreString() should return a StringType,
not a String. This avoids useless conversions from String to AtomicString
when StringType is an AtomicString.

R=haraken
BUG=323739

Review URL: https://codereview.chromium.org/90093002

git-svn-id: svn://svn.chromium.org/blink/trunk@162722 bbb929c8-8fbe-4397-9dbb-9b2b20218538