Add WTF::String::emptyString16Bit.

This makes sure WTF::String::make16BitFrom8BitSource returns a 16-bit string,
which some callers count on even if the source string happens to be empty.

BUG=412032

Review URL: https://codereview.chromium.org/587173005

git-svn-id: svn://svn.chromium.org/blink/trunk@182606 bbb929c8-8fbe-4397-9dbb-9b2b20218538