Remove the ReturnUnsafeHandle parameter from v8String.

After https://codereview.chromium.org/18336004/ , this parameter is never
passed, so we can just always return a local handle. This CL contains no
functional change, just dead parameter removal.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/18106005

git-svn-id: svn://svn.chromium.org/blink/trunk@154209 bbb929c8-8fbe-4397-9dbb-9b2b20218538